Love is always a difficult matter to dissect due to how many different aspects are involved at any one time, which any individual may or may not be aware of. Not only are there infinite different situations but infinite different answers to each due to everyone's varying opinions on them. This makes for a very tangled weave.
I at least, have always seen disaster met when one decides purely on the rationale of another rather than acting upon what one feels is the right move. Often, instincts will not lead you awry where one is at least confident in the matter. How confident do you feel?
Occam's Razor is something that I would subscribe to however. The theory goes that the simplest and most effective means is the best. Therefore, if you wish to know if it is acceptable to date your friend's ex, why not simply ask your friend? Sure, you might not be comfortable asking him, but isn't that another indicator then that it isn't the right thing to do? If you're not willing to risk his wrath with this girl then maybe you should seriously consider a few things.
If you ask him, and are immediately rebuffed, then at least you have received your answer on the matter and need not worry any longer. So long as you treasure your friendship (as I assume you do or else there is little stopping you) then I wouldn't proceed with at least some form of gauging how he feels on the arrangement. A lot of people will tell you that friends will be there when others will not. Conversely, a partnership wherein those that care about each other enough can be a powerful thing indeed.
Let's not get ahead of ourselves though. So far as you've described, you don't seem to be in love with this girl and are at least semi-attached to your friend. What have you got to lose in talking to him? Ask, and then at least you have more to go on as it doesn't dictate whether or whether you cannot go ahead. If your goal is to do this cloak & dagger behind his back, then examine your reasons for this - isn't it a sign that you're doing something unacceptable when you have to make use of subterfuge and secrecy to go about it?
If its any consollation, unless you happen to be ridiculously lucky at this interval and this girl is your life-mate, then I would presume you will not care one wit about all of this in the future as the next love-interest wafts into view (but you may or may not be down one count of a friend). In the end, from all of this, you have to decide what's right and wrong as people are able to deal with different things than others. We mortals are fickle with our morality like that.
Regards,
MultiMasky