Peter Gun,
This is a subject that has been covered a lot over at the Ruger Forum.
Normally to install a cylinder in a single action, all you need is the cylinder. If your lucky enough to find one that fits. Sometimes you can get close enough to have it fitted up to your gun, sometimes not.
Ruger has a policy of not altering guns from their original configuration. If your friends gun was originally a convertable, Ruger will fit a new mag cylinder to it. If it was not originally a convertable, they will not.
Best way to find out is to call the records department and ask them.