| Avianfoo |
Using Bonethrall ability the limit is 1HD per character level. However if the Gravewalker creates her own undead using animate dead there is a limit of 4 HD per character level from all abilities at one time.
| Anzyr |
Gravewalker is very clear about how many HD of undead you control to quote:
"The witch can control up to 1 HD of undead creatures per caster level."
So your player is doing it very wrong. Note however that your player can spam Bonethrall against an undead creature until it fails as it does not have the 1/day/per creature clause of many hexes.
Avianfoo has confused Animate Dead which has its own pool of undead that other means of controlling undead do not count towards. Keep in mind that a Gravewalker Witch can also use Command Undead (which Gravewalker Witches conviently get), which has no control cap. I highly recommend that you use it to maintain control of any undead you create with Animate Dead that would be outside your control limit. Command Undead last days per CL and with a lesser metamagic rod of extend you should be able to control quite the horde. (Honestly, though my primary reason for playing a Gravewalker Witch is Possess Undead which is incredibly powerful.)