Well, I've gone through it, and most will. The only way you can over come the feeling of jealousy is to talk to 'Joni' about it. From personal experience, its easier to be out and open about it then to keep it bottled up, you'll feel hurt and betrayed if you see them and they are actually dating. Talk to her, thats the only thing you can do.
That happens to me alot. I know how you feel. D=