Ask a friend to flirt with her

You know her very well. After all, you've been together for so many years. You know who's her secret crush. Luckily for you, he has a crush on her too. You might be able to use this to your advantage.

You invite Mike on a guy's night, drink some beers and build the courage to finally ask him:
"Hey Mike, I know you like my girlfriend and she likes you too. Why don't you hit on her?"

"What?! What do you mean? She's your girlfriend, man!"

"I know, but I mean... there's so much tension between you... why don't you just blow some steam off?"

"And you're telling me you'd be ok with that? Ha ha, good one!"

"I'm serious man, as long as it's a one night stand and nothing serious, I think it would be quite fun for everyone involved."

"Well, I doubt she'd ever agree to that, anyway..."

"You'll never know until you ask her... But don't be too straight, she doesn't know that I'm telling you this, be as natural as you can."

"Shit man, you're serious?!"