teenager's problem

hello world!!
haha...
im posting here to ask for a solution 4 this friend of mine..
u see..
i hav dis one vry good friend..about 4 months ago he met with dis one girl.became vry close to her in that period.they kept texting each other.and in that short time he fell in love with dat girl.

The problem is dat the gurl already hav a bf.both dis gurl n my friend really love each other.
dis is one thing i don't understand.how could the gurl fall in love with my friend if she already hav a bf?
weird huh..haha..
eventually the gurl's bf found out about her close relationship with my friend.the gurl had some quarreling with her bf..the gurl then told about dis to my friend which made my friend felt guilty.

Now,my friend is trying to avoid n forget about the gurl.he stopped texting the gurl n evrytime they walked past each other my friend did not even say 'hi' to her.he is really trying to forget about dis gurl.he tried to forget and avoid the gurl drastically.

But by doing all this the gurls close friends started to question about my friend's actions.they feel dat it is to cruel.my friend is now lost.he just doesn't know what to do now.he asked me for a solution that could really help him.

But i'm not really good at dealing with this kind of problem.problems that involves relationship.
all i know is that this kind of relationship only brings pain and despair.
haha..i just dunnoe what to tell to my friend.

So what do u think??can u help me by giving me some solutions so that i can help my friend.
he's desperate.
hahaha!!!