… I don't want to show that I am right and he is wrong...
That's because you can't. You're in the wrong.
Yep, he disappears when he has a boyfriend. You know this and need to get used to it. Lots of people do this. Sorry if it doesn't feel fair. That's life in the big city.
As for your "fear" and your saying "if I do not like him" - well, you've already judged the guy without even so much as seeing him.
You are going into this with the expectation that you aren't going to like the new boyfriend. Essentially, you are setting up a self-fulfilling prophecy for yourself. You find some fault or another, say you don't like him, and voila! You predicted it.
Except you're putting your thumb on the scale.
How about contacting your friend, saying you'd love to meet his boyfriend, let's have coffee, my treat.
And then see how it goes. I bet your friend would at least consider talking to you again.
But you won't know if all you do is hem and haw and find reasons not to meet this guy.