I see nothing wrong with it. People will say it makes a child less independant I disagree.
My daughter slept in my bed from birth-6.5 years old. Now she sleeps in her room or on my floor depending on her mood where she feels like sleeping. I was a single mom too. I've never made a big deal about it and whens she was roughly 6years old she decided on her own to start sleeping in her room. Maybe I was lazy, exhausted or just liked co-sleeping but it was so much easier to just climb in bed together then put her to bed and deal with the whole bedtime fights other parents deal with.
My lil brother use to sleep in my dad's bed when he had weekend visits with my dad and around age9 or so decided on his own he was to big and decided to sleep in his room as well.
Maybe since your son left your grandson's been through alot and has had alot of changes in his life and right now might not be a great time to change his sleeping pattern/routine.
I love co-sleeping it was great for my daughter and I. We were always so busy it was the one time of day we got snuggle and bond. My Grandma was always on my butt about it too she'd find a article in the newspaper about it and mail it to me etc. She kept saying I was harming my daughter and my daughter would not be independant. Seriously I can't think of a single negative thing that came from it.
If your just knit-picking your grandson's mom your gonna open a can of worms. You may not like her or who she is but until she's really harming your grandson or neglecting him I'd let little stuff like this go.