Did this girl irritate you before you knew she was your T's client?
You may as well dislike her because of her.

However, talking about her with your T sounds a bit suspicious. Do you really dislike her so much that you want to spend the precious time you have with your T talking about her or is it more connected with her being your T client?
In general I think it's OK to talk about her in your therapy- your T should be able to deal whether or not he knows her or not. Just be honest why.

BTW, how can you be sure she's your T's client?