Just to give you 2c from somebody who has been around the block a few times...
From what I have gathered... your ex decided to muck around on you with your best buddy... and now she wants you back...
Like a lot of good advice has already been given on here such as 'there is a reason you broke up, and you easily forget that when you are lonely' is true...
Even without bringing into question the loyalty of both your ex and your best friend... they both done the wrong thing to you.
Sometimes people fuck up... we are not perfect... its a fact of life. But we also have to live and learn from our mistakes in everything we do.
To give your girlfriend a second chance, you have to sit down and separate your feelings from the matter and look at it in black and white... because only then can you be honest (and you have to be very honest) with yourself if she actually is worthy of a second chance... or whether she must learn a lesson in life just like we all do....
I use this principal in all facets of my life.... if somebody fucks me over, and they are not worth forgiving, then I don't get angry, I dont be petty or seek revenge.... I just simply dont give them the time of day or another thought anymore.... because life is too short to hold grudges or resentment....
Just be honest with yourself is the most important thing.
You cant wipe a slate clean and start fresh again if your are lying to yourself....
So yeah... just sit down.. have a think and work out for yourself if she is worthy.... or life is too short
Good luck champ