Yes, get counseling. He needs to cut things off 100% with her. No contact at ALL! Temptation gets the best of them once they cross the line. My fiance cheated on me with the same woman he cheated with 7 years ago.... on his previous LTR. The second they got the chance, they screwed around, ran into eachother, had a few drinks, and then it continued. and for at least 4 months...
I don't mean to scare you, you sound pretty calm about it.
Just find a good couples counselor, I shopped around.... recommend to ask your primary care physician.....
He needs to figure out why he did it and what he will do to never do it again.
It's like your house burning down, and you just rebuild. But don't realize later that the foundation was bad, and it falls down after awhile.
anyway, that's my analogy, I waited too long, it got so painful....now 9 months later, on therapist #3.