K.S.
I had this situation with my daughter. I really liked the mom and the girls had been friends since 1st grade. They grew apart because the other girl became part of a clique and was mean to my daughter. It did get tricky, and though I tried to just say "it's a shame they grew apart, but this is life, maybe they'll reconnect, etc" it stayed awkward. I think the mom suspected her daughter was mean but didn't want to just ask. So she kept prying for info and trying to get me to say my daughter was a big part of the split (not saying my daughter never did anything wrong, but I wasn't about to let her daughter off the hook). I didn't want to rehash their issues, their was no point. Eventually, we grew apart as well and I haven't spoken to her in a few years. It's now to the point that we wave if we see each other, but definitely not friends anymore.
I would still take the approach of just saying that you hope you guys can be friends even though the girls have grown apart. And perhaps that you should all agree to not discuss kid issues. For some people, this will work great, for others, not so much!