Hi, leg length discrepancy (the official terminology!!) is quite common post THR. If there is a small difference (trying to remember from orthopaedic days) maybe 1cm or less, then they just leave it. If more then you can have a shoe raise fitted. Don't worry about doing anything at the moment...v. early days, things tend to settle once swelling has gone down and your mum has spent a few weeks walking on it.
Has your Mum not had a OT assessment yet, they are normally the ones who do all the home adaptations (like chair and loo seat raise) and check that the house your Mum is being discharged to is safe. She will be D/C'd with crutches so will need to learn how to use those on the stairs (physio will teach her that), plus lots of exercises to do at home.
The hospitals I worked in used to have fairly comprehensive discharge plans post D/C from THR, if you are concerned about any aspect of your Mums care I'd speak to the ward sister, or her named nurse