I would do exactly what sea70 said. Put the feelers out there to see what she'll say, maybe she's been hoping you will ask but doesn't want to come out and say it?
How Can I Approach a Mom About Giving Us Custody? LONG!! - Page 3
I approached a mom about custody and got power of attorney, then guardianship and finally adoption in less than a year. I just said that I knew things were difficult and that he had expressed wanting to stay with us. We said that we were more than happy to do so and would be willing to arrange visits. She agreed.