Hiya
They say the min requirement is 2ltrs, so the fact she is at least drinking that is minimal...and should be ok.....BUT, most people would say try and drink as much as you can up to the 4ltrs.
It is hard at the beginning but funnily enough eventually you will get through your 2 litres by around 2-3pm and then you have another good few hours to get some more down you!!! That is what I have found.
There is a link which talks about the benefits of water and really, if your mum has quite a bit to lose, it helps with skin....it keeps it nice and flexible. Type in the search box,,WATER. I thought I have saved it but I cant find it, but it is more beneficial if she can drink more.
I cant say for definite if she will lose more, but personal opinion is yes, it will, but I cant be sure. I just try to maximise the weight loss as much as possible so I try and go the extra mile to get there.
Hope this kinda helps...if I find this link, I will post again.