The worst part is that this isn't even an ADA violation. The ADA does not cover this (and the normal expectation would be that they use the men's room, which I can absolutely see why they wouldn't want to do).

But the manager was still 100% in the wrong and deserves a nice heap of negative publicity for it...then perhaps her boss will do something about it.

I certainly wouldn't say anything about an obviously struggling teen being helped by his mom in the restroom.