Be direct and honest.
Although Pisces are emotional and sensitive, she will appreciate you being honest with her from the get go. In the long run, after the pain of the break up, she will respect you for letting them know.
Butter him up.
Make sure your Pisces knows how amazing he is as a person, but you just don't think you are on the same level anymore. He has a fragile ego and using kindness and flattery while breaking up might be the best way to go.
If you truly want your Pisces to get up and leave, act distant and cold. If you become a mean person, she will leave you. She only appreciates nice and caring people.
Show him an angry side.
Even if you don't have one, get angry about something and smash a glass or something. He despises dramatic acts of anger and he won't want to be with someone that shows these tendencies.
Tell her you hate animals.
Pisces love animals and usually have many of them. She will not be able to be with someone who can't stand her dog or cat. You will be kicked to the curb before any of their animals will be.