Mostly vanilla, but likely still applies. Honestly, I’m in the same situation and have been working to fix it since it is something that my partner quite enjoys.

I’ve got a one-track mind and have a tendency to just focus on what I’m doing and when I am vocal it’s more light-hearted and laugh inducing. Dirty talk has always felt…awkward to me, but the easiest way I’ve found to transition into talking dirtier is focus on simple “I/You like” statements or observations.

“I know you like the way it feels…” “I love how you feel…” “I love the way you look when you…”

Things of that nature to break the ice so to speak and the rest of it evolves like any normal conversation would. The specifics for your friend will come down to chemistry, but it gets easier the more you do it.