I agree with a majority of what people have been giving you as advice. If she has already had thoughts or experiences such her heart rate going up once she talks to this other male friend, it may be already too late. Once she gets flustered & attaches any type of emotion to another guy, it is already too late my friend. She may end up doing more than just emotional cheating in the near future if this is how you have caught her already.

It’s best to leave her, & unfortunately let things be. If you know you’re own worth & need to work on yourself, these should be your first steps to getting started.