I think an important distinction to make when it comes to feelings like this is whether you are actually jealous, or if you are experiencing envy. They can often times feel extremely similar, the difference is where the emotion actually comes from.
To me, jealousy implies hatred, anger, and resentment. You see that someone has something you don't, it makes you resent them, it makes you feel like you deserve it more than they do, it makes you feel upset and angry that you don't have this thing.
Envy on other hand usually comes from a much more positive place. You see someone has something you don't, and instead of feeling anger or resenting that person, you wish you could just be in their shoes for a bit. You wonder how your life might be different if you got to do that, or at least just see how life would be like from that perspective. Envy comes from empathy, while jealousy comes from anger.
Understanding the difference between these two emotions, and figuring out which one you are actually feeling, can help give you a better perspective of how all of this makes you feel. If it's actually envy, you know you don't view your friend as some object of affection, and her boyfriend as a threat, but rather you view both of them as people and just wish you could one day experience the same thing.
If you are actually experiencing jealousy on the other hand, I would say that is a problem. Your friend does not owe you anything beyond things in the platonic realm, and her boyfriend is certainly no "threat" to you, because your friend is not interested in you in that way in the first place. Feeling upset and angry over something like this just feels extremely unhealthy.
From an outsider perspective, this is a situation where it's plainly obvious that you just need to let go. I'm not saying you have to let go completely, the reality is I don't think that's possible. I think no matter what, you will pretty much always be "interested" in this friend of yours. I mean, that is how attraction works after all. You can't magically stop seeing someone as attractive once you see it. One of my friends is super interested in someone, he's not into her at all, she even tried asking him out and he said no, yet she still likes him. I think this is fairly normal.
It becomes unhealthy when you structure much of your life around this. For example, if there was another person who "flipped" your switch, and you refused to pursue that on the basis that you still seemed interested in your other friend? I'd say that's an issue. You will probably always be willing to do more with this friend of yours, on the sole basis that you find her attractive. But you should definitely NOT allow it to control every aspect of how you enjoy your time with her. If you find it impossible to do this, consider reflecting on the points I mentioned earlier. Figure out what emotion you are actually experiencing. I have a feeling that might help.