Not quite sure what you are referring to in regards to metabolism, fasting itself will boost metabolism and greater fasting periods will boost metabolism more. Fasting will likely be the “metabolism boost” she needs.
If she’s working night shifts, one thing she may want to research, or you can research for her, is eating according to circadian rhythm. With a turbulent schedule, there is probably a lot to be learned from what she eats at specific times of the day. Her biggest challenge will likely be the additional stress an uneven schedule puts on her body.
The Circadian Code by Satchin Panda is a great read. Also read up on the Argonne Anti-Jet-Lag Diet. It is a way of using food micronutrients to adjust your melatonin production.
It’s good to keep your intermittent fasting flexible - so on the night shifts her schedule time she eats changes that’s okay :) Start 12:12 and then slowly shift to smaller windows add in walking - 10k steps and boost that up and change will happen your moms still young !!! Good vibes for success!!!!
Obviously I’m not 56 years old but I’ve struggled to lose the fat and keep the fat off. When I started to fast, eat healthier I also decided to strength train. I think that definitely helped boost my metabolism and now I have a higher caloric intake but I don’t gain the pounds. This does take time though. It’s not an instant thing