With getting PR via the normal routes, once you have it and if you have it without any restrictions, you're free to do what you want i.e. work or not work and work at what you want to. However, with the LTBV, I believe you have to continue to slog at it for quite some time as they monitor your progress to make sure you're complying with their regulations and making a success of the business. It's certainly not a quick fix to get PR. If your mum wants to continue to run a business for several years, that would be OK. I've got it in my mind that they monitor you for 5 years, but I may be wrong. Certainly the original LTBV is for 3 years so I don't know if they keep an eye on you after that.
I guess, there's always the risk, if the business doesn't get off the ground, the opportunity to get PR could be snatched away again.
Edited to add: In
here on page 20 it does mention about the 5 years for evaluation although it seems to imply it's an evaluation of the LTBV category itself rather than the applicant's business.