sure it was a person and not an animal? they have little regard of graves being animals and all.
if it was a human, they could be drunk, they could be someone who didn't like your mom.
magickally i know more about menstrual blood than urine, but some claim it can be used to break a curse, or even create a curse. while i still feel it's more someone being mean, you might wish to protect your mothers grave just so it doesn't get vandalized. [few spirits linger near there grave, so if you wish to cleanse it as well you can, but i would cleanse/protect your home more than the grave as any spell is more likely to effect the living]