I mean, if you're okay with a boy dressing up and acting like a girl, you should probably be okay with a trans boy doing the same thing.
Besides, if you were realistically trying to hook up with or get closer to someone who had a trans best friend and you were like "eh, actually I'm not cool with trans people" they'd drop you as fast as Hazel did.
You don't need to romantically or sexually pursue Robbie. All he is asking if for you to accept that he is a boy.
Because what else would happen? What else could I do in that scenario?