Yeah, the Muppet Wiki says:
"In the late 1970s, possibly before or after Minnelli's Muppet Show taping, Jim Henson, Carroll Spinney, and Spinney's wife were at a restaurant. According to Spinney, in 2011, they happened on Minnelli's table and Jim stopped, planning to talk with her. She looked up briefly, before returning to her previous conversation. Henson and friends talked with Alice Cooper and John Entwhistle, further along in the restaurant, instead."
And I must say, no matter who it is, there's no excuse for not being kind to someone else; it's one thing if you were disrespected by the other person first, sure, but I think we can all assume that Jim, Carroll, and Carroll's wife did and said nothing wrong to Liza. So shame on her.
Nice to know that Alice Cooper and John Entwistle were cool, though! And I guess John was a fan and/or friend, because unlike Cooper, The Who never worked with the Muppets as far as I know!