One of my users in multisite want to buy his own domain but

Hi guys,

One of my users in multisite want to buy his own domain but to still stay in my multisite network.

In my multisite i got few tricky parts. Every single user, post, comment is showing up in main site via WPMU plugins.

Now user still want all benefits just his own domain.

Any suggestion? Does he need to buy hosting, how to put his domain istead the one his using right now. Can i do that with pro sites?

Thanks!