Thank you all SO MUCH; really appreciate it!
KDVP - you're bang on the money re 'sluggish bowels' - that seems to have cleared up now, following a higher-fibre diet, and my mum's been fine; energetic, good colour, etc etc; and then the doc. dropped the kidney bombshell! (Is there any chance kidneys can improve once the bowel problem seems to have been sorted, in your experience?)
You - and everyone else who suggested it could be caused by meds - also prob./possibly bang on - she's been on meds for her heart for some years now, following an attack of costochondritis, and irregular heartbeat. Because of the meds, she's been having regular blood checks for kidney problems, etc, which is how they now discovered there's a problem. However, the GP assured her the kidney problem WASN'T connected to the meds.
I'd quite like her to come off the meds, at least for a while, but there's always the risk the heart problem will come up again. GGGRRRRR. Don't know whether or not to believe doc - is v. likely kidney probs could be caused by irregular heartbeat(?)
Will def. be checking out what to do and not to do with her GP/pharmacist (and if it's OK, will definitely be following the advice given here).
Is also interesting re fluid intake - my initial reaction would have been to drink more water, flush kidneys out, etc, but have also heard that's not the thing to do.
Clarice08 - no, it's not lupus, thanks, though. I hope your Mum improves! Will send her a prayer, if that's OK. It's so frightening when it happens, isn't it ...!
dogwoman and goodhealth - thank you so much for your tips - really appreciate them!
Thank you all again xx