It was rude of him to ask you to take it off in my opinion, and he would not have been able to tell it was not a diamond even if you had taken it off! I have fooled many jewelers with my moissy and have not corrected them. I even sold my first one so I could buy a larger one and never told them that what they were buying was not a diamond!!
What the...?! So you're saying you got more than you paid for it?
To the OP, I wouldn't be comfortable taking my rings off unless it was someone I was really close with. I would never even dream of asking a close friend if I could try her ring on, but maybe that's just me. Even if I did allow it, I wouldn't worry about anyone figuring out it's not a diamond. Most people can't tell the difference.