Unless she plans on taking it on the bus to Angkor Wat or something equally adventurous, having a laptop in Phenom Penh is no different from having one in any other city. You definitely don't want to buy anything that runs hot, naturally, but it's not like you need a Toughbook or something. She should consider getting a quality surge-suppressor (you can tell the good ones because they weigh a lot for their size, but she probably only needs a couple of outlets) and a good lock (which of course you need no matter where you leave a laptop). You can pick stuff up in Bangkok or Singapore too, of course.
At that price point you're mostly looking at the non-Thinkpads from Lenovo -- ie, Ideapads, etc -- and they don't have quite the sturdiness for which the Thinkpad line is so beloved. But I haven't used them in the real world so I don't have any idea; I'd expect them to be roughly comparable to the better-built HPs (there's a fair bit of variation amongst all the major makers, especially between their "business" lines and their "consumer" lines, the latter of which tend to be cheaply built to shoe-horn in more flashy crap at a low price-point).
You could easily get a dual-core Thinkpad x100e for that price, but that is definitely into "ultraportable" territory. (Though it would be my choice -- the non-glossy screen and Thinkpad keyboard make it far nicer than anything else in its class or even much larger machines). I assume you're looking in the 13-14" range? How important is battery life? Does she need an internal optical drive, or can she live with an external (or without one altogether)? (WRT to that last point, note that -- at least the last time I was in PP -- there's a lot of CDs available at very low prices. They're not waving the skull and crossbones, but it's pretty clear what's going on.)