Hi sarah2323, welcome, and I can certainly appreciate a little of what you're telling us - I had a friend at school who had to remain hidden from his girlfriend for similar reasons...
Personally speaking, although it's great that a site like this can bring people to overlook at least some aspect of their religious divides - ultimately in real life there are plenty of boundaries, and I really don't see what you individually can do to overcome them.
In this instance, Karan has clearly made his choice, and if his mother were to have such strong feelings about the situation now, it is unlikely you can do much to win her approval.
I'd hate to say it, but if he has moved on, there is little else you can do but move on.
Hey, I know it sucks - but it's something I think most people go through and learn from and come out of stronger for it. It hurts now, and you'll remember that - but I'm sure there are better situations and experiences for you in life ahead.
Hope that helps a little...