Maybe she isn't explaining very well or maybe he is not believing what she says. At any rate, there is definitely communication problems here that need to be worked out ASAP if this relationship has any hope.

He would be uninformed because its a very obscure situation to be in in the real world.

I'll admit to being bias, but I have lots of ace friends. Granted I am already part of the queer community and I take your larger point that allo people would have trouble understanding this, but we are less obscure than you might think. There is a lot of shame and stigma around saying "I don't want to have sex", most people take it as an invitation to "fix you".To speak nothing of the boundary trouble allo/ace relationships have. I don't know you, but I'd bet hard cash money that someone in your life is shades of ace.