About to get married and starts cheating

So my significant other has a sister who is getting married to a highschool sweetheart. Now hes sacraficed a lot for her, he gave up a scholarship and insane jobopertunitys. Like being a big shot for colgate! But lately hes been screwin up alooot, for instance, he grabed her crotch in mid public. Plus hes been loseing his romance.

Now this other guy, is in her perspecive amazeing and everything she wants. Hes 21 , a 'pirceing blue eyed virgin' who to be summed up in one word from her is 'perfect' So she started to make out with him recently, and she wont take it any further (so far) but he showed up to her weeding communion today, I dont think its right for her to stay with her current sweetheart, but cheating aint coo either xD.

What do you think she should do?

That is typical...spending all your time with one person in high school, not being able to see what else is out there will leave some with questions and curiosity. Obviously, she needs to talk to him about it, from their it's their decision on what to do.