I work in the IT department for our company and we use a MDM tool with our iPhones.
The phone should have some kind of profile installed on it which will allow the company to control it.
this is easy to check if you go to Settings>General and then under the VPN option you should see a Profiles option (note that if no Profile is installed then the Profiles setting wont show).
You can then just delete the profile from there by tapping remove but some companies set passwords so the user cant remove the profile only the admin can, if so just ask the guy to get it removed.