"I don't want to tell her this because I'm worried about her reaction - will she feel attacked? will she get frustrated and leave? etc."

That seems like a clear sign of total therapy failure. If the client has to hold back painful emotions for fear of upsetting the therapist, what is the point? Now the client is taking care of the T.

I kinda thought that the exchange of money gave the client a bit more leeway to act out, or be upset, even if it appears unreasonable. And conversely the T should contain such impulses more than the typical person. Maybe the T should refund payment for every session where she behaved badly.