This isn't about walking away from the ex, it is about walking away from the daughter. I've seen a lot of men who prefer controlling women, it means they can be their ADD selves, but things still get done.
When the guy left his marriage, he also left his daughter. No matter what went on, I'm sure her idea is that dad deserted her and mom. At this point any checks should be going directly to the daughter. The mom needs to be out of that loop.
It terms of the name change...I'd stay out of it. If you must tell, it is probably better to say something like "I heard your daughter is using her step father's last name, what's up with that?' Because, really at this stage it is just an internet rumor. Young people do stupid stuff.