The actual law is here Title 18, Section 922, g(5)(B): https://www.law.cornell.edu/uscode/text/18/922 I would guess that it's not enforced very often.

If your friend was visiting from a European country that didn't require a visa to visit the US (most of them) you'd be in the clear.

Some discussion for non-immigrant visas about halfway down this article: https://reason.com/2019/01/12/illegal-aliens-guns-and-strict...