I don't think you can ever get back into a relationship with a cheater.
For one thing, the only reason the "cheater" is coming back into the relationship is because he/she has no one to go out with in the meatime, so you're like the spare tyre.
I have been cheated twice before. It sucks, and we remain just as very distant friends. It's hard to think of them as nice people because they cheated on you, and hence it is not possible for me to ever regain trust in them that a relationship should have.
Your friend would be better off finding someone new because he/she will not be able to find it in him/herself to trust the cheater enough to start a relationship.
Your friend knows that it shouldn't happen with the cheater, he/she just needs you to confirm that.
*typing "cheater" and "he/she" over and over again just starts sounding funny, reading it over sounds even funnier."