It is possible. You can try to do it at the router level (using a router that supports it), or doing it at the software level (installing software onto each computer). The method mainly comes down to whether you want to buy a new router, and whether you have consent from the other party.

An example of a router that supports this is the TP Link TL-WR941ND.

Software-wise, Netbalancer Free Edition (aka Unregistered edition) supports global bandwidth limiting. I'm not sure if it supports "password locking" to stop your other party from tampering with the settings.

Note that with Netbalancer, it will limit all traffic coming in/out of the computers including LAN traffic. If you share files between the computers, that traffic would also be limited to 5mbps.