I would vote for the Nokia 1020 or 1520 (selling at the same price with Singtel contract). My 1020 seemed to be lost in the sea and I have cancelled the order... and will be getting the 1520 next year (a couple more days). The camera is one of the best... good and stable Window 8 OS, having a huge battery (3400mAh), cool colour (I am aiming for the yellow, but at the moment it seemed that yellow is out of stock... black or white are okay too), big screen (one of the biggest in the market - 6' for the 1520).
If your mum doesn't like big phone, then the 1020 would fit the bill... smaller 4.5 inch LCD, responsive and fast. Best camera in the market (matched only by the Samsung S4 Zoom). The only problem I would have with this phone is the small battery (2000mAH)...