These feelings may be more common than you think. I have experienced similar feelings more than once. I just enjoyed the friendship/didn't worry about losing them. In many cases, it was just a brief phase (one or two months) anyways. I never regretted not telling them how intensely I felt. I even wonder if I felt this way simply because I was "swinging" from depression into an up (euphoric) phase in my life.
I think it is a good practice to tell my husband, friends and family how much I appreciate them on a regular basis. So I would just tell him/her how happy their friendship makes you feel but totally respect the relationship that they already have. Your friend is blessed to be in love and have a friend like you. Friends make our lives wonderful!