Pennsylvania is an "at will" state. This termination is not based on a characteristic that would make it wrongful.
Those would be race, pregnancy, religion, gender, family status, disability, age (if over 40), whistleblowing or having a GED instead of a high school diploma. It's also not allowed to fire you for refusing to do an illegal act, doing jury duty, taking military leave .... Etc. There are no categories into which this fits. (Although the only way to be sure is to check with a lawyer)
This is probably legal. Your friend likely has no protections.
This is clearly not an equitable firing, and its motivation is pretty clearly just the boss's personal convenience. While there's nothing to be done (in Pennsylvania) to absolutely guarantee this sort of thing won't happen again, a good start for your friend would be to avoid mixing their professional and romantic lives in this manner again.