Whoa there! Breathe. That's right, take a deep breath.
I think your mind has been racing ahead to all possible scenarios, but you might want to take it one step at a time.
First of all, you don't know if your mutual friend would want to get involved. The first thing would perhaps be to figure out how to broach the subject with him. Second of all, if your friend is interested, you can start out by asking your husband if it would be all right to just go on a coffee date with him alone. Ask if it would it be okay to just hold hands? Have a kiss? Take it slowly -- you might find that the attraction passes, so why think you have to jump into bed right away just because you realize you feel this way for him now? Let your husband feel like he is part of it, since he has already expressed a fear of being cast aside while you two go do your thing. As Hubby slowly gets more comfortable (or not), you then have a choice to make about whether or not to take it to a sexual relationship.
What's the hurry?