Hi Amy, making decisions around reconstruction can be very difficult and overwhelming so it is wonderful that you are working with your mum to better understand the options available.
If you haven't already done so, take a look at the Reclaim Your Curves website (reclaim your curves). From your description, it does appear that your mum is being offered either a Lat Dorsi reconstruction (latissimus-dorsi-flap) which uses the back muscle and an implant to create a breast, or a TRAM or DIEP Flap reconstruction, both of which use a flap of skin and tissue from the stomach, but in the case of a DIEP, they do not use any muscle. You can find out more here: (tram-flap) or (diep-flap). There are Fact Sheets on the website about all the procedures and some hints and tips about preparing for reconstructive surgery.
I had a DIEP Flap reconstruction in April 2014, two years after my mastectomy. It was a 5-6 hr operation and had a good 6-8 week recovery. It's a very long and complex operation which also worried me but your mum's surgeon will be able to advise if she is a good candidate. I'm very happy with the outcome of my reconstruction and because it is entirely my own skin / tissue there is no ongoing expansion /issues such as those associated with implants.
Best of luck to you and your mum. Jane xx
