If the ass is clean it is safe. I have done this several times over a number of years and I have never had a problem. You can catch hepatitis from feces but you can catch herpes, genital warts and HIV to from any opening in someone's body.
I've always wondered why I wanted to do this so bad but I've always been an ass man and actually have no desire to eat her pussy. I would say if you think it might be unsafe, You Might as well just no do it.