I like others idea of getting them to spring for a dedicated computer. You just have to make it palatable to their accounting department. Maybe lease a laptop and expense the payments or something. If you have an accountant, I'd consider asking them for suggestions on "good ways" on how to make your client pay for your laptop.
In my case, I just outright said "hey, you guys I really want you to be my client but I'm gonna need a new laptop". So we bought a new laptop as part of the contract.
Presumably the client will terminate your contract with them if you don't comply. So you don't actually have to install the agent until the day that they terminate the contract which is also the day that you no longer have to install the agent.