Is it ever acceptable for a partner to tell you who you should or shouldn't be friends with?

No, but its fair for your partner to give his honest opinion if he feels your friend is manipulative or a negative influence. Although he must also have examples to back up his opinion. I once had a friend who was really manipulating, but I couldn't see it... until it was pointed out to me. Then I changed the way I behaved with her and she cut me off. Problem solved..