In the Gardens: during the concert
"I won't argue with them, but she's not for me either. I feel for Mum. She likes Warbeck less than I do and she's stuck sitting through it." Fabian felt Celestina Warbeck was a pale imitation of Muggle artists in the same genre and not up to standards for wizarding music either. His own mother being a composer, he had strong feelings about that.
Rodolphus might well have heard his speculation to Rabbit that Molly liked Warbeck just because it annoyed their mother so much.
"And I get to come out here in the relatively fresh air and do anything else."