Regarding what your M should or shouldn't say, my advice is to let her decide what she wants to say. Stay out of it. If he wants to continue the R he has with her, great. But, it is no longer for you to get in the middle of.
I had a similar sitch where my H reaches out from time to time to talk to my SIL (my brother's wife) who is probably my biggest confidante. I have mixed emotions about them talking, but realize it is not for me to decide what kind of R they have now. It is not within my control, nor should it be.
Just my 2 cents.
BTW, I love your new attitude about wanting your kids to have the best possible R with their dad they can. I agree that is very important too.
M- 18 T-21 S-14,11 & 10 BD 6-18-2012 (OW-EA) H moved out 11-3-2012 10-5-13 Me- I want a divorce. I want to move on w my life. 11-25-13 Jointly filed.