I have a guy from my past , a few actually, that could get my head spinning in a nano second. I also tend to hold fast to feelings forever so if they were ever there they can be rekindled immediately.
I am also incredibly new at poly myself so may not have the best reply/advice.
Why not take it day by day and keep communication open between all parties. I do not think you need to define it or where it is going just yet. just enjoy what is happening and check in with your husband and your friend often to make sure everyone is comfortable and feeling good. And while your in lala land with your friend do make sure to devote time , affection and love to your husband. My biggest fear so far about poly is loosing what I already have with my partner. So I think its important to nurture what we already solidly hold.