Tough one. Using the rule , ‘if you can’t say anything nice don’t say anything at all’, I’d probably keep out of it.
However, if asked outright I’d probably start with, ‘I think you could do better’, and see where the conversation goes. He might possibly agree given the opportunity to be frank.
Although, if he/they don’t realise things are amiss from the audience reaction, well, they might be beyond help!
Good luck.