I'd try and look at the situation dispassionately and try to work out why my former best friend wants to move away from a strong friendship. It might be due to that person or other factors. Then you should figure out what to do.
First of all, I wouldn't be dishonest with myself and brand this other person a "friend stealer" and label my ex best friend as blameless. Does that really make you feel better? Really? It wouldn’t me. I’d be mad at my friend. Of course, I’d get over it because I know what comes around goes around …seen it enough to be convinced, but I’d be mad for a little while, anyway. The truth is if your friend left you high and dry for someone else, he/she betrayed your friendship. Your friend isn't a puppet, is he/she? Does your friend have no say or choice in who he/she chooses to hang with? Your friend made a choice. Give him/her credit where credit is due. Be honest with yourself and your healing will come sooner and be more effective when it does. I think you are smart enough to know that friendships cannot be stolen, only things can be stolen ...unless of course you are talking about an actual kidnapping, involving a gun, threat, blackmail, rope, cuffs, duct tape &/or other subversive or aggressive devices or strategies. However, I think you know that is not the case, right? You give this "friend stealer" too much power and your friend (or ex friend) too little. Pick better friends and be the best friend to them that you can be. That is the only thing you can do. I would like to offer this hope, though. I hope you don’t let this diminish your trust and benevolence toward prospective friends in the future. There are plenty of people out there who are devoted, first-rate comrades to those they dub friends. I hope you learn how to recognize them and avoid the ones like your new ex friend. God speed and good luck!