Here’s an idea, and perhaps a sales pitch too.

Plug the switch side of the jack directly into the modem. Basically modem to jack. Does the speed increase?

If it does, but not up to the modem speed. Then have a CAT6 or even CAT7 handy that is roughly 100’ long. Run it out, don’t keep it coiled up at least, and plug it into the modem. What is the speed? Then plug it into the switch, what is the speed?

If the 100’ CAT 6 or 7 maintains the speed. Then use that knowledge as a sales pitch... but advice them on the mess that is involved in rerunning. So hopefully he’ll just deal with it. Or choose the CAT 7 for future proofing.

KOT