Most Helpful Girl
As long as they are both interested in one another in a serious way, it should be fine. And if they don't work out, as long as it ends amicably - without any cheating, lying, violence, etc. - it may be a bit awkward for a bit but should be fine.
Do you trust them to be able to date without damaging your friendship?
Most Helpful Guy
She'll probably drive a wedge between you and your buddy, but so would any other girl tbh. If you try to stop it you look like an asshole, it's a lose/lose situation I'm afraid.