I really feel for you. I was in this situation with my grandson and it was horribly difficult.
I agree: Trust your instincts. And I would pray for her and your grandson and all of you. I would also go to Goodwill or whatever and buy a bunch of stuff for her child . . .If could get a ton of stuff, wash it, and send it to him. I hope there are other adults in his life who can watch over him.