Everyone in your cabin has to be on the same package anyway, so it's only the fact of buying drinks for friends who didn't sign up for the free drinks package. Staff are very good at spotting it when you order and advise you don't or just bill you for the second drink to your account. So not worth it.
Either your friend is worth a couple of quid (£/$) for a drink or they should buy the package and enjoy like you. You can get the package mid cruise to make it cheaper if that helps.