crushing hard

so there is a new guy i like he is very thing i would want in a realtionship but there one BIG problem.... he is my closest friends ex. 

I have no clue what i should do she says it would be okay if i was to date him.  i don't belive her though on top of this problem the guy doesn't know that i like him. I don't think he would go for a girl like me but i don't know if i should even find out because of my friend