(...though I'm not sure why he would be calling him Fosse when his friends, Gwen included, always referred to him as "Bobby" but that's another story.)
It's definitely Bobby.
In their only screen pairing, Verdon and husband Bob Fosse dance the comic mambo, "Who's Got the Pain?" At the end of the number, Tab Hunter (Joe Hardy) says, "That was terrific, Fosse!" Verdon explained, "That was an ad-lib that they kept in."
Actually she doesn't say differently. She acknowledges they left in the ad-lib. The rest of that sentence is someone else saying stuff.