If your friend puts in enough details this could be on-topic
The problem is that there are many possible scenarios that you have to think of. For example: are we talking about the police having only got an anonymous tip that they should search an area the size of a house? Or are talking about an area of a graveyard? What resources does the esearching party have? Just humans? Specially trained dogs? Are we talking about a current-day setting? Or for example near-future? Is there a time limit involved? "Never" is quite a lot harder than "it's not reasonable a body will be found in the span of a couple years".
There are lots of questions that your friend would have to answer. In theory it could be on-topic, but it could just as well be off-topic. For example a simple "How deep you would have to bury someone so they wouldn't be found?" is lacking details and therefore seems to be too broad for me. I recommend that your friend writes a draft and you put it in the Sandbox. The problem is that you need 5 reputation minimum to post on Meta, so your friend won't be able to post it here, unless she first asks/answers/edits something on the Main Site.
A little warning though about this specific topic: there are quite a few people who don't like gruesome topics. Look at the torture tag for example. Depending on how the question is posed you might need to expect a higher chance that someone will downvote such a question. Again, the Sandbox can help with finding a way to phrase such a question.
See also: Should we be mindful of potentially dangerous questions? and Should questions about painfully killing people be welcomed on this site?