The working of the friendship between you two should not be done through a spell; not in my opinion, at least. Since you two want the friendship to stay and very strong, why use a spell to do what you two can do on your own? If you want it then work for it. You will feel much better about it by working on it with him. And even if you did a spell to strengthen your friendship with him, you two will still have to work for it. It's not just going to happen with the only effort being a spell.
Arckh, if you love her yet she is with some one then do the right thing and let her be with him and be happy. If it's truly meant that you two are to be together then Fate will have its way of bringing you two togethep, but I'm not saying not to put effort into it. However, presently, it may or not be a good idea to go ahead and tell her how you feel. That choice is up to you. Just think it through and be aware of the many possibilities before you make your decision.