I'll admit that I did not yet listen to those recordings, but I'm not really good at identifying equipment just by the sounds, anyway.
Yamaha has had so many keyboards over the years that this might be quite a challenge. Do you remember if the keyboard was a new or current model for the time, or was it the case that your friend's sister had the keyboard for a while? Because the PSR-300 was about 7 years old by 1998. By then, there was the PSR-320 and maybe even the PSR-330. And of course, there were also the PSR-4xx and PSR-5xx versions of those boards that were significantly more advanced. However, if it did just have a couple of red LED digital displays for numerically identifying sounds and styles, then it was likely an early 1990's (or earlier) keyboard, because starting with the PSR-320 (and 420 and 520) or so, Yamaha started supplying more advanced LCD data screens with these models.
You could also check out the PSR-400 and PSR-500, which are the more advanced versions of the PSR-300 -- I did some quick Google image searches, and it looks like those keyboards did have a drum sound for each key for the drum-kit voices, and not just on the white keys. The PSR-310, 410, and 510 were kind of unique in that their numeric data keypads were laid out linearly, with all of the numbers in a horizontal line, instead of the more common telephone or calculator style layout found on most of the other models, so that would be something memorable about those keyboards.