Train simulator games don't make any particularly stringent demands upon monitors (e.g. they don't require particularly fast response times). If it was me, I'd simply go for a decent 17" refurbished monitor, costing the princely sum of thirty quid:
http://www.accomputerwarehouse.com/product-info/dell-1703fp-refurbished-tft-screen
(NB: That's not just a random web link. That's the company that I most of my computer equipment from and I can thoroughly recommend them).
However if the games require a widescreen monitor, his budget will have to increase for something like this:
http://www.accomputerwarehouse.com/product-info/aoc-e2250swdnk-21-5-inch-widescreen-led-monitor-1920x1080-5ms-vga-dvi