It's doubtful that your modem hardware is the cause for the poor gaming.

It's possible you may be experiencing a limited NAT type which could impact your gaming experience.

If you're connecting to your internet through a wifi connection you should change it to a wired connection if possible

If you haven't already, I'd put your console in a DMZ. This will open up all ports to your console and remove the majority of impediments you could be experiencing with your connection.

Dropping a couple hundred bucks on a high performing modem is unlikely to drastically improve your gaming.
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There's a bit more to troubleshooting latency than just buying a new modem. Problem could be inside his house if he is on wifi, or congestion on the shared network for his area/neighborhood. What troubleshooting has he done so far?
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That's a good point.

If there are multiple devices connected and you don't have a decent internet package you will struggle.
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Does he have a wifi router sitting behind the modem?

There's a TON of stuff that needs to be checked to help with latency on cable.
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Actually, I'd think that the modem would be the very last thing that you'd check. There are so many other larger opportunities for failure.
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It's almost always the router. Whether it's NAT settings or the need for QoS.
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