It would depend upon how close the friend is to me and what exactly you mean by the alleged “catfisher” is saying or doing that is misrepresenting him or herself.
It’s one thing for the “catfisher” (if they are actually doing that) to say that their name is Rex and they own a dry cleaning store, when actually their name is Rex and they work at a dry cleaning store. In this case, it will be known to your friend soon enough, and really isn’t that big of a deal, unless they are being horribly mislead about how much money they have (and are planning to support your friend, financially).
Another scenario is that they say their name is Rex and they are pursuing your friend, romantically, and you know that their name is actually Ted, or even Barbara. I’d want to know why the person is lying about their name, and possibly their sex. If I knew the friend well, I’d probably ask them about this discrepancy and tell them how I found out.
Another scenario is that the person says their name is Rex and they are physically fit, in good health, and trim, but you have found out (and it needs to be certain, not just something a third party told you) that their name is indeed Rex, but they are in bad physical shape, very unhealthy (maybe a druggie, or they smoke, or they have an STD), and they are morbidly obese. I think I would tell my friend, because he or she is most likely making some type of romantic overture towards someone who is not anything like what they have described. If your friend tells you she doesn’t mind those things, then no harm no foul, anyway.
Another scenario is that the person says their name is Rex and that they are single. If you know for a fact (not just that someone else has told you) that Rex is married, or involved romantically with someone else (if your friend is expecting someone to be exclusive with her and has told Rex as such) then I think I would tell her.
Another scenario is if the person says their name is Rex, but you found out for certain (not just because someone else told you) that their name is Todd and you know that Todd is taking gifts and money from your friend, and your friend thinks that they are in an exclusive and loving relationship with who they think is Rex, and they think Rex is a great person who will also be sharing money and gifts with her, this would probably be the biggest red flag of them all. I would definitely tell her. One of my friends (who is a very intelligent, and resourceful woman, got catfished, but she figured it out on her own, before any money or bodily fluids were exchanged. I only found out about this particular situation after she told me, not the other way around) In another instance, and this was years ago with a much younger friend, her supposed “exclusive boyfriend” lived with another woman. I found out and tried to gently tell her, then he caught wind and told her himself in a roundabout way and broke up with her, but she blamed me for the breakup, and never believed either one of us that he was living with this other woman. That friendship ended on that day. So you have to be prepared that the friend might not believe you, even if you have proof.