I think you should be very happy that you have found such a wonderful potential home for your pet. If you trust these people and feel assured that your horse would be well cared for with them, I don't see how you could feel bad. As already mentioned, you aren't doing her any favours if you are struggling to keep up her medical needs. They are only going to become more and it will be difficult for you not just financially, but emotionally, when you are not there for her. If this couple will take her, your pet will have the love she is used to and deserves.
As for not having a friend... any transition will be difficult, esp. for an older horse, but it could be that she may make very good friends with the couple. For quite a while I only had one horse here, and it went quite well. She would wait for me at the end of the paddock when she heard my car, and call for me. She was a very snobby horse to people b4 that, but became very loving after losing her pasturemate. Also, many horses are quite content with the barn dog, chicken, goat or whatever. Or watching the activity in the garden. It just depends on the life at the new place.
It sounds like you have been very good to your horse and I think you should continue to do so. Be proud.