Your "friend"
Right...lol
Anyways, I have some experience with girls. I as a male, can't tell you the mysterious ways of their thinking or reasoning. But I am 19 years old and have been in a couple few relationships in my day.
I've never been in a relationship with a childhood friend (all mine are males) but maybe he really does think he loves this girl due to the bond they have built over their years growing up together.
I can 100% guarantee the girl doesn't love him though. She keeps leaving him for other guys and comes crawling back to him (your friend)
My advice is tell your friend to man up and stop being a little bitch. He sounds beta as ****. He's going to let this girl keep dumping him for other guys and STILL take her back every time? She's probably a ***** and having sex with these other guys. And to top it off he comes CRYING to you?
This friend of yours needs to grow some balls and man up.
Tell him to stop putting up with her **** and that he doesn't really love her.