Hi monkey'smama, I've had a mastectomy - am happy to answer any questions you have
I had my lymph nodes removed separately afterwards (so not exactly the same as your mum)
I was in hospital overnight, - but she may stay on for a few days it varies between hospital. If she comes out early, she may well come out of hospital with a drain (a tube from her wound into a bottle to catch the fluid) and this will be removed probably by a district nurse once she stops producing fluid. The drain is a bit of a nuisance as it gets in the way and you have to carry it around.
She will be fairly sore for 2-3 weeks, and won't be able to lift things or drive.
The lymph node surgery can leave you pretty bruised and with a certain amount of nerve damage, - the hospital will give her exercises and she needs to do them religiously several times a day to get back the full movement in her arm. She will always have to be careful about lifting heavy stuff with that arm.
She will also have to use gloves for gardening, cleaning etc. because with no lymph nodes if you get a cut its much more prone to infection. (But actually I haven't had to many difficulties in that area - its just something you have to be aware of)
She's probably already got a breast care nurse, she will give her a 'softie' which is a spongy soft artificial breast she can fit inside a pocketed bra (or safety pin into an ordinary bra). Once her wound has healed well she will be offered a proper prosthesis - which is shaped and more weighty so like a real breast.
I'd say if possible it would be better if she was with someone for a few days rather than on her own, because she'll feel very tired and won't feel up to getting herself meals etc. It is quite likely she will be under a district nurse for about a week if she is only in hospital overnight - they will check and change her dressing and check/remove her drain, so there will be a daily nurse visit to factor in.
If she does go home on her own, - a few freezer meals she can just shove in the oven would be useful.
Any little treats and sweets would be nice 
I found baby wipes useful - you can't have a proper shower or bath with the dressing on, so its nice to be able to refresh yourself with a wipe.
For hospital she will need PJ's or a nighty that buttons down the front so it is easy to undo for medics to examine her.
Loose clothing for when she is up and dressed again - button up shirts are easier to get a sore arm in and out of.
I wish her all the luck in the world for her treatment and recovery.
I've probably left lots of stuff out so please just ask away and I'll do my best to answer questions. Also I post frequently on the Tamoxifen thread here in health - lots of us on there have had breast cancer and mastectomies, you'd be very welcome if you wanted to pop in and ask for helpful tips 