Without control over the client device or the network router, the only solution is to introduce a middling device that you do have control over.
Access Point
The easiest device to introduce from a networking perspective is an access-point as they are designed simply to extend existing networks rather than provide packet routing. However, bandwidth throttling is considered an 'advanced' feature in these types of devices and is, therefore, quite expensive. In fact, I don't know of any consumer access-points that support throttling.
Second Router
So, I suggest you get a second router. There are two ways to do this depending on the features provided by your ISP router:
ISP Pass-through
If your ISP's router supports 'modem-mode', then enabling this will allow you to setup your new router as the sole router for your LAN. You can then deploy a router with the functionality you want whilst still using the ISP router as the modem. However, not all ISP routers have this feature.
Multi-Layered NAT
A more universal solution is to create a new subnet, like so:
Network B Network A
192.168.2.x 192.168.1.x
Client Device ------------------- Router B ------------------- ISP Router ----- Internet
192.168.2.87 192.168.1.2 Public IP
You can then connect any devices that you want to throttle to Router B and configure the bandwidth limits from the Router B management interface. Devices that you do not want to throttle can still be connected to Router A.
This solution involves multi-layered NAT; this has some drawbacks as discussed here. However, a home network is unlikely to encounter any issues.
"my only ethernet cable is super short" You can buy longer ones ... :) – DavidPostill – 2016-10-09T08:06:16.710