I don't have any experience myself, Other people here have been elders and seem to have good advice.
I am in my 30's and my closest JW girlfriends, 2/3, were married to UBMs. After they had kids... they felt they had to try extra hard to raise them JWs. They subjected themselves to their service overseer or random elder in the hall who took interest, almost as if it was their husband. Not sexual in any way, but in a bizarre way I can't explain. There is some common bond between JWs being IN vs OUT... he is capitalizing on this.
I would play it this way-yes its a game to play. I would talk to HER and make sure she knows you feel its weird. I would act like you are threatened by their communication--it being inappropriate--why isn't the WIFE texting her anyway? Her wanting to give a 'good witness' will see this is stumbling you and be more sensitive to YOUR feelings, than to defending him. Make sure she stays on YOUR side, not defending him. Who cares if he is ugly, fat, bald...i am sure your wife is young since she had a baby...and affairs rarely happen due to good looks. My point? Use this to get her on your side and have him back the F*&^ off. Its weird- and totally inappropriate.
I am in my 30's and woke up after multiple back to back pregnancies, because of the amount of work vs lack of support from congregation members. I hope this works for your wife.