In answer to the OP: no. The fact that she had sex with this guy and the fact that you both agreed to an open-relationship have nothing to do with this question. If you two are truly trying to rebuild your marriage, then neither of you can have such a strong friendship with anyone but each other. If you convince yourself otherwise, then you will guarantee failure. And since he is such a good counsel, the 3rd party should understand this and get out of the way.
With the sort of problems you are describing, it is unlikely that either of you is responsible for more than 50% of the blame. Assigning more blame to the other person is an obvious recipe for disaster, but assigning yourself more than 50% is really just as bad. Neither party can start with the moral high-ground.
One easy thing you should do is quickly get interested in the following:
Finally, I think the posts by Heart of Dorkness and Hamshire were very well put.
Good luck!