I can certainly see why you would say you are worried. You refer to her as not being sure who is has feelings for. That statement in itself suggests that even if they are friends she is considering the prospect of starting a romantic relationship with him. A relationship can't survive with you if she is questioning whether she has romantic feelings for him. She has to decide where her feelings are in terms of which person and pursue that as a relationship. Right now it seems that she is sending mixed signals to both of you and that is not fair to either of you. You may want to take a step back and decide if being with someone who isn't sure how she feels is productive. If you continue this path then she may be able to have both of you on the hook. It would be interesting to know what she has told him. Maybe he doesn't know she has feelings for him. Maybe he doesn't want a relationship with her and the feelings are one sided.
If she is telling you that she is having conflicting feelings then the relationship is in trouble anyway. I would start having firm boundaries with her so that she doesn't just go from him to you. Tell her what you will and will not tolerate. If she is devoted to you she will be with you but within the guidelines that you set. I always say that your intuition is most times right. If you feel like you can't imagine yourself without her then that is saying no matter what you accept her behavior. Take a hard look at what that could mean. She could be acting sweet but you need a commitment that her heart is in the right place.
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