Yes, all the resorts will do a vow renewal. Check the website under weddings/honeymoon. You can get some information there. Then contact them and they can let you know if they will allow a single on. I have seen one or so for weddings, but I think they limit the number of singles allowed.
Couples does offer a vow renewal. You can find it on the website under the weddings section. My husband and I had our vows renewed last December. It was so easy and enjoyable.
Couples allows singles day passes at the Couples properties. If your single friend could find someone else to travel with it would be a lot easier. They do allow same sex couples.
The vow renewal is $200. You could choose either the beach or garden gazebo. We really enjoyed the experience and are headed back for our 4th visit to Couples.