Is it possible to make a server running Windows Server 2012 R2 Standard a member server of an existing domain running on a Server 2008 R2 Foundation Edition server?
According to Introduction to Windows Server 2008 Foundation on TechNet:
In the Windows Server 2008 R2 Foundation operating system, the server must be a member of a workgroup or, if joined to a domain, joined at the root of the forest as a member server or domain controller.
This implies that other non-Foundation servers can be part of the same domain, but can anyone confirm this is true before I instruct a client to purchase the server license?
Yes, you can add a member server to a domain that is using a Windows Server 2008 R2 Foundation as its DC and "primary" server.
To "try before you buy" and eliminate the worry of burning money, you can download a free evaluation copy of Windows Server 2012 R2 and then purchase a product key after you confirm that it works.