You can ask her not to tell you about it......
Also the one paying rent is you and if you don't want him around perhaps you can agree with the roommate not to bring in any male companions?
Or if he is around you can make plans so you are not in the apartment when he is there.. of course this is less than optimal but.... you have control of your life....
Perhaps you could be grateful you are NOT her?
In any case I know about daily triggers and working on my own feelings has been the best way to move forward, I think after your mourning is over it will be way easier to dettach from this or any other addict that you have to interact with for some reason.
Be gentle with yourself and remember its YOUR home also, and you are paying to enjoy your own safe place.