Yes.
If you care about them that much, what are you so worried about? That she’s gonna like licking snatch and run off with the other girl? If I’m dating her, I know she’s not crazy or dramatic because I don’t tolerate that shit. Don’t be selfish. If she’s willing to share you, the least you can do is share back. Hell, maybe she’ll even get jealous halfway through and start beating on the friend. Even that wouldn’t be so bad.
Most girls are gonna have a 3some at some point or another, but most of them do it with someone they’re “not emotionally attached to” because they have sharing issues. You gonna let some random dude swoop in and get more from your girl than you after you put all this effort into it?
Oh wait, this is nyspeed, i have to put it in terms everyone can understand. Suppose you had a ferrari and you were in love with it. Then someone you know offers to let you take out their lamborghini for the night. You’re still gonna do it. Change of pace FTW.