What's ur gut say?..I'm always learing of so-called "best friends" that rat out friend, to me there's always a hidden agenda...And lo and behold, she's got one ... a big one ... she's had the hots for you lol...
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Ok first things first,..the exgf, did u have any concrete evidence of her 2nd time cheating?..Anything beyond her bf telling you?..If so, then u made the right decision...If not, then there's a possibility that the bf might be feeding off ur insecurity in this relationship to get her foot in the door...
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Either way tho, I'd say u should take some time in between getting over being deceived (or feeling like there was always the possibility) and starting a new relationship with the friend.....
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She's waited this long to come clean with her true feelings for u that..I'm sure she can wait a couple of months so you can get your head together...
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As for the ex, if u feel it might be true then you've kinda lost the trust in that relationship anyway so try avoid looking backward..

Stay away from both of these women, unless you have a burning desire to meet Jerry Springer...

Your ex actually started the suspicions by first cheating on you. She shouldn't be surprised when it bites her in the ass later. Stay broken up with her, with no trust your relationship is over no matter what you do...
As for the friend, you may as well sex her. At the very least, it will piss off your ex, although you may not be the vengeful type...

I don't think your ex's friend made it up, but what's done is done, you can't trust her anyway so it's best to just move on (in my humble opinion).....
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If I had to guess, I'd say your cheating ex is from Martinsville, IN...

It's hard to say. Did you have reason to suspect before that great friend of hers stepped in? Seems odd that you were together that long and wouldn't have had any previous indication that she was fooling around, but I'm sure that kind of deception has happened in many other relationships before. But I think you might be better off avoiding the whole situation entirely. Maybe the truth will eventually come out, maybe not. If after time apart, you find you really miss your ex, it might be worth revisiting...