I think you're probably right...typically the guy does the asking and the woman has the right to turn him down.
To be honest, though, I'm not sure which is easier. It can be excrutiatingly difficult and painful to turn someone down nicely and without going into great detail as to why you don't think it would work, particularly when it's someone you really do like a lot as a friend.
Just don't use that 'I don't want to spoil the friendship we have' nonsense. It would be nicer to just kick them in the junk and walk away.
And BTW, women most definitelty have the upper hand..and I write from the perspective of one who, over the last 5 years, has watched my daughters set their caps for two guys, and then "allow" the men to woo, win, and marry them. I'm crazy about my two sons-in-law, they're great guys, but they never had a chance..never saw it coming..
If you really like him a lot as a friend -- I mean a close friend, not just a "hiya" friend -- and as the kind of friend you trust and respect, you shouldn't turn him down. You should marry him.
Marriages based on strong friendships are the kind that last.