Follow these steps to understand the possible issue for the server using too much RAM:
Take a look at his server, what's the actual RAM usage? The clients sometimes open a ticket since they're playing with a friend, but since the RAM is at 4.5GB/6GB, they think it's too much.
At first, let's focus on the RAM being maxed out or close to being maxed out.
First, take a look at the view-distance and simulation-distance. The default would be 10 (chunks). Sometimes the clients have changed it to absurd amounts, and if that's the case, ask them politely to change it to 10 (or depending on their server's RAM, to 15/20).
If the view- and simulation-distance are at 10 as it should, take a look at their plugins/mods/datapacks. If there's a lot of them, that might be the problem. For Paper servers, it's easy to figure out which plugin it is by running "/timings report" or "/spark profiler open".
Duper/Farms, so there is a lot of entities in one place/chunk. This is what the console pops up with sometimes, if that might be the case:
[05:23:25 WARN]: Can't keep up! Is the server overloaded? Running 15916ms or 318 ticks behind