I am taking an introductory course on South Asian History and the professor was going over the origins of Indian Civilization, particularly the origin of the Aryans. The professor was very clearly in favor of the "Out of India" theory, as he said that the Aryan Migration theory was unfairly treated as dogma by the academic community, and one of the reasons he gave for this was that the Celtic Druids of Europe were descended from the Druhyus, a tribe from the Rigvedic period. Is there any evidence to support this, or is this just pseudohistory?
There is zero evidence to support this. Even a cursory search on this topic returns nothing but extremely dubious "sources." As far as the historical linguistics, the "Druhyus" and Latin druis, genitive druidis are completely unrelated. We have no idea what the Celts called their priests—"Druid" is a Greek-Latin term, from drus "wood, tree, oak tree" and the -idis suffix "offspring of." Cf Old Church Slavic dru˘va, Albanian dru/drushk, e.g. The tribe "Druhyus" is almost certainly from the PIE root dhreugh^2, "deceive, harm," giving us Old Indic present formation drúhyati "seeks to harm"; Old Persian imperfect adurujīya "was deceiving"; Avestan družaiti "lies, cheats"; Old High German triogan "deceive"; and for all the Skyrim fans, Old Norse draugr, "ghost, one who cheated death". The two are separated by geography, time, and basic linguistics.