There could be a PHP Shell which will allow him to do pretty much anything he wants even look at httpasswd files etc..
So Check everything out before allowing people on. also theres a program called "TeamViewer"
www.teamviewer.com
Which allows people to remotely access your PC if you give them a password.
You can also cut them off when you want and change password. I Recommend using that and let them install what you can see