Originally Posted by knot_sweet
Honestly there are too many variables to easily answer this question.
You would probably need to establish the ground rules based on your desired outcome.
Do you want to punish your sub by setting her up for failure knowing she's aroused servicing your friend?
Do you want her to do it purely as an act of service, knowing she won't enjoy it so you can reward her for fulfilling a fantasy of yours?
Do you both get off on the idea of her being a slutty toy at your command?
How does the friend fit into this? Is she submitting to him as well as you, or is he just along for the ride?
What plan do you have if it goes badly?
A lot of the answers depend on the D/s relationship already established and an educated guess about how she's going to react based on open conversation.
There are a couple of submissives here who's dominants instruct them to play with others. But the differences in the dynamic between those two relationships are pretty extreme.
In a nutshell, it's down to you two. We can tell you about our experiences, but it probably won't help much with what works for you two.
Thank you for the well thought out response. To answer your points in order:
No, it is not to set her up and then try an punish her or just my own fantasy.
She loves to follow my orders and finds the idea of being ordered to please someone simply because I ordered it and she gets pleasure from my pleasure.
As for the friend, he would just be on the receiving end of anything and not be someone in control of her.
In all, I am really looking to see what was the reactions between others and the idea interests me so I want to see how well the subs and doms that have done this reacted to the situation.