Objectively, it's a weird thing to go after someone who definitely had sex with your best friend. Eskimo brother be damned! Wait, is it still kosher to use the term "Eskimo"? Wait, is it still kosher to use the term "kosher" when not talking about food blessed by a rabbi?!
Think of the conversation as a PowerPoint presentation. Walk them through your feelings, explain your intentions, and really try to convey that you're not just looking for sex and legitimately can fall in love. Still... tread very, very lightly and acknowledge that what you're doing is a huge social faux pas.
Follow your friend's advice closely
So, maybe your friend is kinda cool with it, but has some reservations. Whatever you do, do not ignore those reservations. If he’s over her, but will need a few months to be fine with the concept of you two as a couple, try your hardest to give him space.