How would an 11th-level NPC cleric save a PC from this?

[Jon_Dahl]

One of the PCs is loyal to an NPC cleric, whom he regards as a mother. The NPC cleric has asked the PC to eliminate another NPC, who is a high-level wizard. This wizard lives a very secluded life in her tower and only contacts a handful of powerful individuals from time to time, none of whom are loyal to her. She has no friends. The PC is determined to carry out his "mother's" wish and kill the wizard.

Now the wizard has indirectly invited the PCs to visit her in her tower. She needs to collect materials for a powerful magic item, and a friend of her acquaintance has recommended the services of the PCs. I think that once the PCs meet the wizard in her tower, it is likely that the PC will murderhobo the wizard. And I think he will be able to do that.

Once the NPC is informed that her protégée has murdered her rival, who had done nothing wrong except that she tried to compete with her in production of high-quality alchemy and thus reduce her client base, she will try to find a way to make it look like an "accident". Or something of the sort.

This 11th-level cleric of Boccob has access to DM fiat, because I don't want the PC to be hunted down. But what would be the best solution to make it look like that the wizard had slipped in the shower and banged her head, resulting in twenty-or-so cut wounds and severed limbs? Of course the body can be destroyed, but is there any way to prevent anyone finding out the truth using magic?

[denthor]

Not really.

Speak with dead in reality only yields yes or no answers. If the character is named specifically it reveals.

A 6th spell almost no one takes legend lore cast on the spot of the murder would yield the name of the wizards murders if any of them are 10th level or more.

If the wizard has a familiar that remains on this plane of existence that can talk and retains the information to spill to others. It must be eliminated if an imp then the soul is taken and contact can be made to keep its mouth shut. Plot hook.

Random murder is done in a secret location with no witnesses is about as anonymous as you can get loot the tower and hope no comes knocking.

No friendships, family or allies it could be months before it is discovered. As good as it gets

[Vaern]

There's an old discussion regarding whether a corpse is a creature or an object, and the consensus often ends up being that it is both but it depends on the way the spell is interacting with the body. If it affects the corpse only physically it is generally treated as an object; if it channels, conjures, or communicates with the spirit of the deceased then it instead treats it as a creature.
The argument could be made that Make Whole could affect the corpse as an object, removing any wounds and restoring any missing flesh. With no visible marks the cleric may be able to pass it off as death by natural causes. Divination could always reveal the truth, but it shouldn't be an issue if there's no reason to suspect foul play to begin with.

[Crake]

Pathfinder has the spell "Sculpt Corpse" which allows you to remove physical evidence of foul play, though it does allow a will save if there is reason for suspicion.

[Mechalich]

The best way to prevent investigation into a murder is to make it look like there never was a murder in the first place. If the deceased lives in seclusion and a brief absence from their social calendar due to being dead is likely to go unnoticed, then the simplest solution is to replace the deceased with a double for a while, a few months would do nicely, and have the double "die" via natural causes and move on.

Hire and/or Quest a Doppleganger to play the part, or Planar Ally something with similar shapechanging abilities.