For every out-of-place artifact or contact hypothesis, the first step is to have the claimant cite their source. Almost always it is a second (or third or fourth etc.) hand re-telling of the original source, often with details added and removed along the way to make the story more dramatic and compelling. These hand-me-down sources rarely cite the original claim directly. At best they make reference to some individual person or source which can than be sleuthed out to discover the original claim amidst a tangled web of re-tellings.
In this case, however, there's only one source positing Pre-Columbian Vikings in the Pacific Northwest. In 1926, one Olaf Opsjon of Spokane, Washington claimed to have deciphered paintings on a rock which had hitherto been considered to be Indigenous. According to Opsjon, the "runic inscriptions" on the boulder told the dramatic story of a group of Vikings who, in AD 1010, were attacked by Indigenous people over a dispute involving use of a spring. The 24 men, 7 women, and a baby made a stand at the boulder, but only 6 men survived, fleeing only to later return and bury the remains of their comrade, now stripped of any valuables (hence the lack of any artifacts).
This is, put simply, nonsense. But it's a good story!
The Spokane Daily Chronicle led its July 5, 1926 edition with the headline "FIND VIKING GRAVE NEAR CITY." The New York Times also picked up the story. An subway strike stole the headline, but the Times still ran a page 1 story on July 6, 1926, titled "Says Norse Crossed America in 1010 A.D."
To be fair, the NYT did run a story the next day which was critical of the find... on page 27. The story quoted Herbert Spinden, curator of Harvard's Peabody Museum, who noted the proposed date would place the events at Spokane just a couple decades after the Norse reached Greenland. Spinden also pointed out that several other proposed Viking objects had been found in recent decades, but none had "withstood the scrutiny of experts."
Indeed, the late 19th and early 20th centuries were a boom time for archaeological hoaxes. There was good money to be made in haphazardly demolishing an Indigenous mound and "finding" some Phoenician coins or a tablet inscribed with Hebrew which could then be displayed or else sold off to collectors and museums. There are numerous examples of "runestones" found during this time -- the most famous being the Kensington Runestone -- all of which have been shown to either be natural or Indigenous marking misattributed to Europeans, or else outright fakes.
Mancini (2002) notes there was a bonanza of Scandinavian contact hypotheses of the 19th century. He posits these stories were a way of counter-balancing the fact of Columbus' voyage with a Nordic/Anglo-Saxon discovery narrative. Thus Northern European colonists had as much claim to the Americas as the Spanish, and could point to their history as one of peaceful exploration, in stark contrast to atrocities of the Spanish which formed the framework of the Black Legend. That this was also a time of increasing Scandinavian immigration to the Americas, and the these artifacts were almost inevitable discovered by such immigrants, also did not hurt the popularity of these tales. A Swedish-American farmer finding a Norse runestone on his land could certainly then project a sense of ownership, both contra other European claimants and Indigenous inhabitants.
Opsjon was himself one of these immigrants, having immigrated to the United States from Norway in the 1890s. His background is unclear, as is what he was a "professor" of, with the NYT calling him a "scientist and expert on runic writings," and he certainly claimed to have authority on the subject. In fact, Opsjon was so enthusiastic about Norse writings, that the Spokane rock art was the third time he claimed to have found evidence of pre-Columbian Vikings (Finley 2005). In 1919, he wrote into the Spokane Chronicle to claim that some rock paintings along the Columbia river were "Runic letters of the earliest Teutonic alphabetic characters." A few years later, in 1924, he claimed to have found a stone tablet inscribed with runes which told of the location of a Viking burial from which he would "exhume a Viking chief in full armor." No such exhumation was forthcoming.
The Spokane boulder was thus Opsjon's third attempt at staking a claim for Viking presence in the Pacific Northwest, and it was presented with as much evidence as his prior two attempts, which is to say without any evidence at all. At the very least he shows development in his stories, giving the Spokane "runes" a dramatic backstory. He even performed an excavation of the site, though it turned up nothing more than some unidentifiable bone fragments, a possible stone hammer, and a flint arrowhead, the latter two objects consistent with Indigenous occupation. A few months later, Arctic explorer and actual lecturer in anthropology at Harvard, Vilhjalmur Stefansson, turned up to the view the site. He examined the runes and was unimpressed, stating they did not resemble ancient runic writing he had seen in Greenland and Iceland, and were more likely of Indigenous origin (Finley 2005).
Unless your friend can put up more convincing evidence, the sole claim to Pacific Northwest Vikings comes from a "professor" who was three times wrong in his claims and who seems to have found evidence of Norsemen under every leaf. In addition to his claims about runic inscriptions and Viking burials, he also stated that Spokane, Tacoma, Idaho, and Sioux were all words derived from Nordic. This is against a backdrop of an explosion in popularity of archaeological hoaxes and fringe contact theories, with Norse explorers being a common theme. Meanwhile, the rock art in the Pacific Northwest, which is of Indigenous origin, is well studied. Tell your friend to pick up Keyser (1992) Indian Rock Art of the Columbian Plateau next time they want some reading material.
Finley 2005 Playing Vikings and Indians. Columbia: The Magazine of Northwest History 19(2), 29-35.
Mancini 2002 Discovering Viking America. Critical Inquiry 28(4), 868-907.
Says Norse Crossed America in 1010 A.D. (July 5, 1926). New York Times p. 1.
Dr. Spiden is Skeptical (July 6, 1926). New York Times p. 27.