In a similar situation, for web hosting, you would use the web edition, which would allow you to connect an infinite number to the server, but that does not apply to VPN's and RDP, especially (unfortunately) since it does not include the file server role.
The way I would approach this is set up a Server 2008 R2 Standard or Enterprise edition, and use device CAL's. It is rare that I would choose device CAL's over uCAL'sCALs, but otherwise he would need one User CAL for every customer. With the device CAL's, he can estimate that he will have 10 users connected at any time, so buy 10 CAL's. The downside is that if user 11 tries to log in, he won't be able to, so estimate a little high.