I woundn’t call that a meltdown, exactly. He froze. He wasn’t prepared at all to “wing it”.
As someone who has been in broadcasting for nearly 30 years, if you can’t wing it, don’t pick up a mic in the first place.
The bad part about that is it says he obviously didn't care enough about the appearance to write & rehearse his own material. He was just going to come in, put in an appearance, read his stuff and go. So yeah, embarrassing for him to be caught taking a lot of his usual audience so much for granted.