well, i have this friend who, i think thrived on "stealing" the guys i liked. she tried to do it with healer but i put a stop to that so quickly cos i was not going to have it happen yet again...a bit off the topic, but there are some people who are ok with it and some people who arent.
personally, i wouldn't date an ex of a friend but its up to those three people on how to handle themselves in that situation.
and if all three of them are cool with the situation and there's no weirness between them, then by all means, whatever makes you happy.
like, if my friend has to start dating one of my ex's right now, i'm sure there'd be that moment of weirdness but in the end...at least she didnt end up with healer