www.sheldonbrown.com has your answers.

single speed freewheels has a ratcheting mechanism build in - they let you coast. so you can't use one to make a fixed gear. you'll need a track cog. the problem is, the wheel your friend has doesn't have a reverse-threaded section for a lockring. you can wrench a bottom bracket lockring on to the threads that you thread the track cog on, but it's not really doing a whole lot. some people add red locktite, which bonds the cog to the threads, as an extra measure - if your friend plans to use reverse pressure to stop, or wants to learn how to skid, this isn't the most secure method. a rear wheel with a hub made for a track cog and reverse-threaded lockring would be advisable in this situation.

of course, even a track wheel doesn't really help if the cog and lockring aren't properly installed.