Hi Lup,
How about just saying No, I won't do that (help disguise the gf from the GF to get the rent).
How about saying No, I think I'd like to stop having these phone conversations about the kids with you, because I want to respect their privacy.
??
I'm sorry this is so hard, I know it is. I just wonder if you can make some decisions about how much access (phone time, etc.) you will give this woman.
It's not going to help you be closer to your son, if you have no boundaries in place with GFM.
Can you change the subject and keep changing the subject? Like, ask her questions about her OWN life, if you feel you must talk to her? Or better yet, be too busy to be talking to her except now and then, and on more superficial topics?
xo
Hops