Can he ask out her friend, knowing how she feels about him?

Can he ask out her friend, knowing how she feels about him?

"I’ve been friends with this girl for about ten years.. I know she’s into me, but she knows that I don’t feel the same. She has a new friend that I want to ask out, but I don’t know if that’s in bad taste.
Is it OK to ask her out?
How can I do it without being mean to my friend?"