Most Helpful Girl
I think it is odd that she told this guy she was unhappy. That is a bit too friendly for me. I think you should leave her and take a break from it. There is a huge difference between platonic conversations and intimate ones. This to me seems intimate since she made the comment about smiling about him and telling this guy personal information about your relationship with her. I think you should also mention that if she has an issue within the relationship, she needs to talk to you directly about it not go and tell others. If you don't want to leave, tell her she needs to stop contacting him and to block him on whatever she is using to contact him on right in front of your eyes. If she eventually unblocks him and continues to talk to him then you can justifiably leave which is what I would do.
Most Helpful Guy
Your making excuses for her actions. You already know she's got feelings for someone else. If you want things to work on. You need to talk to her, and tell her why things aren't working between you two. There's got to be something that's missing, that she likes with him. What is that? Is it, how he treats her, what's the feelings she's got with that guy, that she doesn't have with you. You've got to get down to the brass tax, cut and dry, and figure out if you can work out your issues, she's got. Because there's always something that drives us away from who we care about, and who where committed to.