*is there a reason that its a bad idea to try and tell a narc that you suspect they are a narc?
is it just pointless? or is it worse than that?*
IMO it is pointless, but it is worse than that.
To see that a close friend or partner has a personality disorder can be interpreted as something you have this "on them" or could even use against them and they will not be best pleased. They may also argue with you about it, which is a waste of your energy. Even worse, they will know that you know now. So they will probably "up the game" in a new way, to try and win and to get that power back. Something you don't want to get into, willingly or unwillingly. Finally, they may just hate you even more, because you've got something that they haven't got (insight) and you got there first. In some kind of way, they may want to make you pay for this! Its a bit or a dark view I guess but they can be dark, as we know.
Another thing, if they do gain knowledge of their own narcissism they may think its funny or actually boast about it (publicly at least), even if its presented in a twisted or inaccurate way.
Finally, they can't really change. There may be one or two self-aware narcissists out there who do want to change, but they are rare animals, and I think they probably spend their lives struggling with the issue. Even in the highly unlikely event that your N was like that, is it worth being involved in all that and the risk it entails. Self-aware Ns like Sam Vankin (or whateverhisnameis) don't want to change.
I would add that if you feel the need to inform them of their problem to "help" them, you are probably still involved, when your priority should be healing and looking after yourself.