I'm told that there's one particular RMB bank note that is basically worthless, and that shops/restaurants etc always try and fob theirs off onto foreigners and then won't accept it themselves. Unfortunately I don't know which one it is - anyone know?
I lived there for a year and never experienced such. They also have coins that, if transferred to GBP would be absolutely worthless but, still, I never had a problem using them at any point.
Actually, it was sometimes a problem to spend their highest valued note of 100RMB at the smaller street-side shops who don't have change early in the day and it was always met with an annoyed 'sigh' from a cab driver, for a low fare. So I always tried to make sure I had at least a 50 with me for the cab.
As for taking foreign currencies over there, you can, but I don't see much point in doing so. You can use Hong Kong Dollars in some shops in the south of China (where I lived) but a lot of places simply refused to accept them from my experience. The supermarket chain 'Ren Ren Le' will happily accept them, and even sometimes the McDonalds would be happy to take them.
If you go to Macau though (who have their own currency too), it wasn't a problem using HKD or RMB.
One thing I would advise you tell your mother, is not to allow anyone to take advantage of her money and tell her to bargain like crazy as they really do raise the prices for foreigners. Speaking a little Chinese can help in these cases, as they think you may simply 'know the score' and give up a lot more easily.
Always make sure the cab driver starts the meter before he takes off, or he'll try to rip you off, and if she's not going to Beijing or Shanghai, don't expect the drivers to know any English.
And never assume he'll accept HKD. Ask first. Most won't, but they'll give you change in HKD but you can refuse it and ask for RMB. If he says he hasn't got RMB, tell him to go get some.