I need advice! (kinda complicated situations)

I have two friends who are online dating and they have been dating for about 2 years now they met each other once bc they live in different countries. So one of them asks me for advice bc he started to like like another girl in his work place but is still in love with my other friend. He's not that sure about his feelings for work-place-girl and so he doesn't know what to do and he's been feeling like shit these days and the one who is there for him is the-work-place-girl what should he do in this situation?