Poor, poor father...
Before I get any questions about this, no, up until now Noah's mother didn't know about his crush on Rex. But his amazing and very convincing
lying skills, made it easier for her to get it. And the reason she knows Noah was actually referring to Rex is pretty simple: he always asks her if he can go out with "a friend" when he wants to hang out with Rex. Simple as that.
Sequel to this
(wich is a sequel to this