Didn't the child choose the tutu? Does a young child know that it's "girls clothes"?
We're going deep into gender science here. The overall result of these studies appears to be that you can't in the long run force a child to be something he or she isn't, and attempts to do so are invariably damaging. If, in that case, the parents forced Max to wear a tutu, shame on them. But, unless you were there when the tutu was actually bought, and chosen, you can't really say one way or the other, can you. Did Max point to it and indicate that he wanted it? Was it bought just in case, and taken out, along with equivalent boys' clothes, and the choice was made then?
Definitely the parents need to be careful that they aren't steering Max into choosing his clothes, behaviour and toys to be girls' stuff all the time. I'd say that we need to be careful to jump to conclusions. For us, on this site, it's unlikely to matter a jot. Anyone, though, who had the opportunity to meet Max and his family ought to be careful to jump to conclusions and voice them. That could also risk doing a lot of damage, too.