What server was it on? Look up the morgue file, and you should be able to figure out what he did.
However, without looking at it, it almost certainly wasn't killing monsters indefinitely. Score is based on three things: runes, time, and experience. In that order. Only, if I was giving them a ballpark percentage of how much they matter for your score, it would be something like 70% runes, 29% time, and 1% experience. It is really impossible to ever get more points from killing things than the time it takes you to kill those things. There's also a cap on how many points you can get from experience, and even way before you hit the cap, it starts to get diminishing returns. Crawl is highly opposed to scumming, and it hasn't ever worked to my knowledge except for one time:
http://crawl.develz.org/morgues/0.9/xom ... 165025.txt. There was a bug where you could dupe maurice and clone his inventory to copy everything he held - and he would steal your things, so you could feed him anything. Aside from that, scumming will never net you more points.