I think the best thing to do is to observe and gather information about what is making the other person to steal your friend. It your friend does not care and crosses to the other side, then it means that you are the only one who valued the friendship.
People cannot "steal" your friend because regardless of who your BFF knows, your relationship with them is still the same.
If they are trying to separate you and your best friend, the best thing you could do is go up to your friend and say "Look, I feel like so-and-so is trying to push us apart, and I just want you to know that I value your friendship and don't want anything to ever come between us."
I'm not sure what you deem as 'stolen' .. Are you referring to someone else trying to befriend your friend? I'm not sure how that it 'stealing' your best friend..Best friends are friends because they choose to be friends .. not because that's the best they can do at the time until someone better comes along. People can have lots of friends .. not JUST a 'best friend'.
Your friend will have other friends .. and those friendships should have no affect on YOUR friendship. However .. if this other friend is turning your friend against you .. your friend is dumping you or avoiding you .. then you have to re-evaluate what your friendship means to your friend. If they can be manipulated like that, when it comes to how they feel about you .. then, I'm sorry to say .. they are not really much of a friend. Good riddens.