So I have been with this guy for 5 minths now. We had a really good relationship and things were going really well. Then all of a sudden he started not showing up when he would say he's going to and not call. When I wouold ask him about it he would say that he didn't want to make me mad because he couldn't come over, so I told him that it makes me mad when he doesn't call and just doesn't show up. Then his best friend and my best friend broke up. Ever since then things between us have gone doen the drain. He repeadtly asks my frined out and when I ask him what's going on he gives me some flimsy excuse. He was told by her that I would have a problem if they got together and he asked her to ask me because he didn't believe her. Then when I asked for an explaintion he lied to my face. And when I go to leave him he shapes up and stops doing things that I feel that he shouldn't like asking my friend out. So right now its at the stage that he just asked her out, and I haven't said anything to him so here's where I'm not sure what to do should I confront him straight out? Or Could his friend be behind all of this to ruin us (because sometimes he acts like he has absolutely no clue what we are talking about)? Or should I just let him do his thing and back off? Is the flirting with my girl normal for guys?

No, its not normal for the guy to flirt with your friends, that means that he's wanting to cheat on your (lets hope you have awesome friends and they shut him down).

You should confront him. If he lies (which it sounds like he has before) then you should just dump him, straight out.

No one deserves to be cheated on, or to have someone attempt to cheat on them. You don't need that, he doesn't respect you and you should find someone else who does.

If he makes excuses now (and its only been 5 months) and is trying to get with your friends, its only going to get worse. I'd end it, make no contact and find someone who appreciates you.

It's awful being put in a position that may bring up jealousy and tension between friends...
What is her position on this? Does he like him? Have you talked to her?

He sounds like he doesn't know what he wants... I agree with the above comment. This is completely disrespectful, make sure you know the information you're getting is true, and if it is then you can do better.