He'll be able to overhear Retha discussing Marcellus Ver. Though it's difficult to make out everything she's saying over the noise, she appears to find speaking about him disdainful.

Retha is very pleased to receive a distinguished visitor and is flattered by his compliment. She will accept his invitation, and bring him to a resting area where wine is served. Technically, as a well-known performer, she often receives drinks on the house at Epulum...but she is quite happy to let Nikolai pay for it and makes no effort to dissuade him.

As a gossip, it would be possible to get her talking about rivals. What would Nikolai tell or ask her about it in particular?