You're right, he's wrong. I think the facts are black and white.

I have a 9 year old and she has a kid's Specialized mountain bike, but I'd never even consider clipping her in and was completely surprised that anyone would even think about doing that to a child. He's being irresponsible and she will get hurt.

As you know, you're in a tricky situation since you're not the mom. But, I think you should intercede. It could hurt your relationship, but on the the hand, maybe he'll understand that how much you care about his daughter.