Peeing into her anus is great fun if you are in a kinky mood.
Experience has taught us to do it before a good bum-fuck. Afterwards she is so loose for a while that the pee won't stay inside. Doing it before anal sex, however, requires an interruption of the act which usually is a shame. It can be nice to include the "into-ass-peeing" in some kinky foreplay.
Greatest fun, however, is in fact after anal sex while still inside her and she likes the feeling of the very-well-tempered filling. Try it in the shower, bathtub or preferably outdoors. She can totally relax and let it flow.