In the European Middle Ages, medicinal practice stagnated and didn't progress much beyond that of the Egyptians, Greeks, and Romans. Monasteries were the semi-official centers of herbal medicine through that time and were where you would find hospitals and hospices (source).

If you had access to a monastery, chances are you would encounter someone who was well-read on Egyptian/Greek/Roman medicine and would potentially have been aware of some of the ancient Egyptian diagnosis and treatments like this described in the Ebers Papyrus:

“Shouldst though examine a patient with stomach disease suffering from pain in the arms, the breast, and on the side of the stomach, say: 'Death threatens.” And if though examinst a man for illness in his cardia, and he has pains in his arm, in his breast, and in side of his cardia, and it is said of him: It is [w3d] illness, then thou shalt say thereof: It is due to something entering the mouth it is death that threatens him. Thou shalt prepare for him: Stimulating herbal remedies…” (source)

Herbs referenced in Egyptian records for chest pain include basil, juniper, and mustard (source). A poppy preparation would have been used as a pain killer (echoing contemporary medical practice into the mid 20th century), with willow bark probably also used.

Foxglove and other digitalis plants were used for cardiac (and other) issues such as dropsy, with the active compound still used in medicine today. The use and effectiveness of digitalis weren’t officially documented/published until 1785, however, it had been used as a part of traditional herbal remedies. Of import, however, foxglove/digitalis is not mentioned in Dioscorides' De Materia Medica (published in ~65CE) which was considered to be the preeminent guide to herbal medicine for nearly 2000 years (source). Despite this, it featured significantly in English and European folk medicine. Depending on how strictly a monk would adhere to De Materia Medica, a patient presenting with symptoms of a heart attack may or may not have been given a herbal preparation that included digitalis.

In terms of diagnosis, it is likely that a medically minded monk would recognize the described symptoms as coming from a cardiac origin, however, as OP inferred, treatment wouldn't consist of much more than a herbal preparation, likely something for the pain, and prayers to God that they would survive. The Ebers Papyrus above is pretty blunt in regards to the prognosis of people presenting with the symptoms of heart attacks.

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My quals: Bachelor of Arts in History and Classical Studies, Bachelor of Nursing, now a Registered Nurse that dabbles in history.