Wow, that is very considerate of you to offer a refund.
Your friend should not take it personally, it happens sometimes in those situations.
Even though your client ignored her, he knew what he was getting into and paid the full amount.
Thats very considerate of you to feel that he didnt get his moneys worth, but it was his decision to ignore her.
No refund required.

Agree somewhat with an additional twist:

Has this guy done doubles before? Sometimes it can be sensory overload (he did NOT know what he was getting into!) and he can't switch focus or handle all the conflicting (conflicting with normal experience, that is) sensory inputs. Eyes see one empty hole, penis feels it is already filling one hole. Hands squeeze two boobs in front, but back feels two boobs rubbing from behind. Eyes see 2 hands and 10 fingers in front, brain senses 11th digit bringing up the rear.  

There can also be the Monty Hall effect. "Will you keep Door #1 or trade it for Door #2?" I'm feeling really good and got a rhythm going and I do see another honey right over there but what will happen if I give up Door #1 and I don't like Door #2?  

Or, my Big Head wants to try Door #2 but my Little Head ain't lettin' me!

I don't think that a refund is required but I'm pretty sure that if you offered a discount on a solo session that he would be very appreciative of your generosity and that he'd be coming back for more.

I agree with below that refund not required in this situation-  years ago I drove to appointment with well reviewed traveling provider- the assistant after several attempts couldn't get her to answer phone so session never happened - anyway she came back a few months later and I decided to give her another chance-  told her about the previous time- she mentioned that client had brought wine and she got drunk - apologiazed andi insisted on giving me discount for session- I think it was like $50- wasn't reaaly looking for discount but thought it was nice that she did it

On one other occasion I kind of took a partial refund -the provider pics were her- but old and not really a fair representation of what she looked like- so I was a little disappointed- anyway it was a long time ago so I don't remember the exact details but I think I might have booked an hour-  she had a half hour rate on ad also- so I only stayed 1/2 hour and paid the 1/2 hour rate- she didn't complain on one hand felt guilty for not stayed ng booked hour but on other hand thought that her pics were misleading-  though like I said I paid the correct rate for amount of time

I had a date with a provider down in philly last summer. I had made a long trip for me and had booked a few weeks in advance. She had done some porn and I had been looking forward to meeting her. I had booked for 2 hours and left the donation as such. I wasn't expecting a big GFE session, but it was right to business. During round 1 she asked if I could come quickly since she was really sensitive. I had no problem coming so we were cleaning up and she was getting dressed. I asked why and she said that we were done. I informed her that I had booked for 2 hours and told her I'd be taking back some of the donation (gave her a one hour instead of 2).  She said that she could stay if I wanted. I could sense she wasn't into it so I said it was okay. I think she felt bad so she suggested we go get some food. We ended up getting a couple of cheese steaks and had a number of people come up to us while eating saying we were a good looking couple. So, that helped my ego a little bit.

The problem, as other posters mentioned, seems to have been not with you and your friend but with the client and his apparent lack of experience with three-somes.  Doubles are an advanced technique, and require some understanding of the dynamic to enjoy the experience.  It is a bit like Deep Powder Helicopter skiing, while the client may have paid for the equipment and the helicopter, it is up to him to figure out how to get down the slope.  With doubles, I live by two rules:

1.  As the guy, don't try to direct the scene; let the ladies control the proceedings.  They are the professionals, you the guy are not.

2.  There is an additional emotional component to doubles, as one party can feel left out, so effort needs to be made for everyone to feel included.  The trick to a successful three some is that all three parties have an obligation to ensure that everyone feels involved, and I find that this is what makes the experience fun.  That takes a bit more work on the part of all three, the guy included, but that is also what makes for a successful doubles. Otherwise, it becomes a mechanical process.

From your description of what happened, I think it's possible your client got what he wanted. Maybe he enjoyed your GF watching. Since you didn't detect any unhappiness on his part, I wouldn't assume he was not satisfied.

he would be able to tell you more than anyone else