The question has been asked: "What do you say to the performers when you are obligated to talk to them afterward but you didn't really like the performance all that much? Sopranos are apt to say "What a fabulous dress!" Others might opt for "that was a very ambitious program."
If it is your obligation to speak to the performer you must realize that they also know they did not play that well. If you think it is your job to make them feel better let them know you appreciate them coming to play. Nothing you can say is going to make them feel as though farting in a bath tub sounded like the Bells of St. Mary's. If they come at you with excuses then you can address those issues. But, really, in cases like this the less said the better, and offering up empty compliments will do nothing but make you look condescending at patronizing. Vance Wood.