I've never been able to understand why friends can't date ex's. They're EXes for a reason, just because he didn't work out with your friend, doesn't make him undatable for everyone else. But apparently that's JUST me, because hoardes of girls typically jump down my throat saying "OMG, friends are more important that BOOOOOOYYYYYYS" Which I never understood, husbands are more important than friends, which roughly translates into boys (not that ya'll are going to get married or anything) but just saying.
You should talk it over with your friend. Tell her you know she was hurt by the break up, and you will always love her and take her side, but that you really like this guy, and you hope that she'll be happy that you might find love, even if it is with him.
To me, if your friend gets upset, that's really not a friend, but then like I said, girls typically disagree with me on this, so I may be the wrong person to ask.