When the audience is uncomfortable wants to take care of the performer (Stritch, Liza, Cook) it is time to stop the concert level of performance. It's really heartbreaking.
That being said there are fantastic ways to have an audience with a legend, like Cook. Like recent events with Carol Channing, give Barbara a couch, a moderator, some clips and let her interact with fans. She is even welcome to sing as she likes, as much as she likes.
There are ways to honor a legend without pushing them past their capabilities.
"The sexual energy between the mother and son really concerns me!"-random woman behind me at Next to Normal
"I want to meet him after and bang him!"-random woman who exposed her breasts at Rock of Ages, referring to James Carpinello