how to make the jail safe for the parent system?

[Andrey Groshev]

Hi, All!

I understand, what not absolutely normal question, but...

There is I and my server.
Also there is other person a server responsible for a web.
Periodically he wants that I would instal some software, but in my 
representation, this software bad or unnecessary.
I wish to make jail for its and its software.
To give to this person complete access to it, let does all that wants.
But, if in the jail create wrong start scripts, then the parent system 
too cannot be started up to the end.
For example: in jail  in /etc/rc.local write /bin/sh
And that starts all after this prison will not receive handle.

Question: how it to avoid?