I work as a PA in orthopedics, and we do about 5 or 6 knee replacements per week. You mum should be okay to drive after about 2-3 weeks, depending on how she's feeling. Most of ours are back on their feet and driving in that time.

Is she doing a custom fit knee where they do an MRI before hand and make her a custom knee? We've had a lot better success doing those than just doing regular generic knee replacements.