Direct evidence from him, no. But I think it's a reasonable assumption given that it's a stereotype in our culture. If I swapped the genders, then his question sounds completely out-of-place to me. I'm willing to assume that it is sexism because the mental-model I think a person would have to have for it to not be sexism seems less likely.

One could, of course, just ask the person what they meant, but we weren't privy to the actual conversation to know if she did.