There is no cheap answer to this, and nothing appropriate for a 5-PC office. If he thinks that doing this means he only has to buy 1 license for the software, he's incorrect. Any method of doing this is going to expensive, period.
Citrix XenApp (formerly presentation server) was made for this, but you have to have good server-class hardware to run it on, plus Citrix licenses, plus Microsoft TS licenses, plus the software you want to run in the first place. It's also not set-and-forget, you need an admin who knows what they're doing to set it up - although maintaining a 5-person environment shouldn't be a full-time gig once it's done.
Windows TS alone can do this generally well now too, but again, you're paying for it.
Citrix and VMware (the companies) also have VDI products; but now you're talking some bucks to deploy and maintain.
MS has App-V, but again - you need a server (or servers), you need to have the skills to package the apps and maintain the systems.
Novell had/has ZEN, which includes an application launcher, but that's (IIRC) not much different than publishing applications via AD GPOs. So, you could just do that.
Honestly, for maintaining a 5-PC office, just install the needed software on each machine, after paying for it. Unless there's a specific problem that the owner is trying to solve, and you didn't explain it?
