I would ask the friend first and I would ask in terms of what you said here ,
that you think you are interested, you can't say "love" but you are attracted-
but that you want to ensure that EVERYONE IS ONBOARD FIRST becaause your friendship is deeply important to you-presumably more than fucking the wife would be.
You say they are poly-so having someone come to him and say "I'm attracted to your wife, I'm intersted, but I value our friendship more than I'm willing to risk it without your support" should be taken as a good sign that you are aware it's not a 'I'm going to see if I can get her to leave him for me" situation and are being upfront.
It also allows you to acknowledge your lack of experience again-up front and allow him the opportunity as a friend to divulge to you some helpful hints on how NOT TO PISS HIM OFF or disrespect him in the dynamic.
Personally-if a woman came to me and told me that she was attracted to my husband and wanted to know how I felt about her approaching him I would be so damn impressed I would be inclined to set up the opportunity for her.
Furthermore-I would be more inclined to trust her and THEIR relationship and intentions if something did come of it.
Fuck-I'd be THRILLED to have a metamour who came about that way.
And as a wife-if someone went to my husband that way, and my boyfriend, I'd be VERY impressed by that as well-because their RESPECT for my already existing relationships would show me that they had MY BEST INTERESTS at heart.
"Love As Thou Wilt"