Zsa-Zsa drinks quite a bit too, but it was most evident in the last few years of my RB cat, Brando. He drank loads, and preferred outside water to inside but i'm pretty sure that your friend's cat has the same thing as Brando had, and i was even more sure as i noticed he was 15. It'll be his kidneys not working as well as they used to, this is very common in older cats. The kidneys can not filter the blood as easily, so the cat drinks more to flush out the system. Tell your friend not to panick though, Brando was diognosed with kidney failure when he was 8, and still managed to live to be 15, and he was a pedigree british blue cat, so a tux will probably live longer. It shouldn't really affect him at all for a while, he just needs plenty of water available. But if your friend does notice him looking unwell, he should be taken to a vet immediately, everything happens pretty quickly when the kidneys decide to shut down, but if you catch it early then you have a good chance of getting them to work again.

It is, however, a sad fact that many cats die from kidney failure. I won't lie about this, eventually Bliksem will probably have to be put down once the kidneys pack up for good. I don't want to be morbid, and it feels awful posting about this because it brings back such sad memories, but i need to tell it to you truthfully. But, i do expect that Bliksem has more years left in him, and if she's only just noticed him drinking more now, then it should be a couple more years where his kidneys will work moderately well.

I sincerely hope that Bliksem leads a happy long life and that he is never in too much pain.