Why is your friend against you getting another cat? Does she not like cats?
Her theory makes no sense b/c Tigers and Lions live together in 'packs'. Granted I think mountain lions are loners but still. And most of the people her that have more than one cat, seem to have success.
Kylie lived with her brother for a year. Then I got her and she has been a cat living with a dog for almost 2 years. I think she would be fine if she would have stayed with her brother. If she had to choose b/n a cat and a dog she would probably choose a cat!
I thought she would like being an only pet when my RB Shaianne died. But she didn't really do anything. I think she spent alot of time just laying around ( As opposed to now when she does the same thing! )
I think you should go with your vet's opinion and do what you think will make you and Luna happy. You know her better than your friend. And I don't mean to offend you b/c I know she is your friend but is it possible that she is just telling you that b/c she doesn't want you to get another cat? Unless she lives with you or is at your home 99% of the time, frankly I don't see how it is any of her business!!! If one of my friends told me not to get another pet, I would have to consider their motives.

IMHO!!!!