Welcome buddhaisbetter! I won't repeat what everyone else has said here (which is very true). I am currently in this organization, and I have to warn you. You have plenty of reason to be worried.
Even though this woman seems to be rebelling from the Jehovah's Witnesses, her statement that she IS a Jehovah's Witness is very telling. I have known many people that have been disfellowshiped (aka excommunicated) that are still captives of this religion. So although she is doing something wrong by being with another man other than her husband, her beliefs are still largely JW.
No doubt she has been "witnessing" to your son. Even if your son does not fall for this cult there are dangers. I am married to a VERY active Jehovah's Witness. The fact that we have very different beliefs has been extremely damaging to our relationship. Your son will only have trouble if he stays with this girl.
You may want to think about trying to get him into a different counselor. Most psychiatrists/psychologists know about Jehovah's Witnesses. They are a mind control group that, while they seem to have good intentions, tend to cause extreme problems in the family life of non-believers.
While it may sound like an easy fix to go to this womans elders, I do not see this as having any good results. She will get disfellowshiped without a doubt. But that will not free her from her captivity to this cult. It may, in fact, have the opposite effect. She could try to "come back" and try and get your son to support her in doing this. They would have no association at the kingdom hall, but once she got reinstated into the organization they tend to "love bomb" people. It will seem to your son as if the congregation has a huge amount of love. And this could cause him to be fooled as well.
The best thing to do is to try and show him the origins of this organization and show him that it is similar to many other dangerous cults that have proven to be deadly in the past. Maybe then he can either distance himself from this woman, or even perhaps show her where she has been mislead.