well it seems like your friend screwed up. and now she had to live with the consequences. Some people will tolerate (repeated)infedelity, while others won't.

If she loses him over this its her own fault. He can't trust her anymore. She took her current boyfriend for granted when she kissed her old one.

i'm sure there are many other underleing issues here, but tough luck for her. she could have opted to work on them, but instead, she made a mistake. ....hopefully she'll learn.