I think you answered your own question when you said you think Mum can walk 50 meters without aid. To get enhanced rate of mobility in PIP your ability to walk has to be 20 meters or less.
It's entirely up to you whether you want to risk asking for reassessment; I think the probability is that all of her award would be taken into consideration again, and there is always a danger of losing the award she already has. I suppose you could look at her mental state, and her ability to manage planning journeys etc. on her own (see the guides to PIP) but I would be very careful.
Similarly with the length of the award.
Sorry to sound so unsupportive, but we would not like your Mum to lose the award she has.
Look at the guides and make up your own mind; just be very careful.