My understanding of this is a lot like how YeOlde has stated, but with a small addition. Their computers/network will still need protection such as AV, a firewall, etc. As long as the RDP connection is secure, and they still have protection for their systems in place, then they should be good. No information is stored locally, so they wont need everything they had before, but they are still going to be using computers that access ePHI, so the no XP thing may or may not apply.
I've got a client who handles the healthcare insurance, billing, collection, etc., etc., and they have to conform to HIPAA standards all over the place. Their employees work from home, and are given a computer with Windows 7 Professional that actually establishes a VPN upon boot to their server that is co-located for AD/DC, and all other things. The actual ePHI is stored in the "cloud" through another company that provides them a Remote Desktop where they actually perform their work. No client information is stored on these individual computers that the employees use, but they still have an AV on each workstation, that is managed by the companies server. And because the connection is all through a VPN, the server has the firewall instead of the individual employee. From what I've seen, it's not a very impressive server, but it has 2 dedicated ISP's, one is a 10Gbps Fiber, and the other a Cable Internet Connection that caps out at 100Mbps and is used for overflow and as a backup. They moved from XP to 7 back in January, and I was part of going to each of their employees here in Phoenix to do that for them. Since then, we've picked up handling their computers as they still have problems here and there (yeah...pretty much helpdesk support. "Hello IT, have you tried turning it off and on again"). We also provide them the MAV and RMM. They got audited back in Decemeber, and the only thing they were told needed to be done was make sure that the XP systems are upgraded from XP. Again, this client of mine does not actually store any ePHI on their systems, to include the server.