How long have you been on the diet? It can take from a few days to a few weeks to transition from burning almost all glucose (from carbs) to burning a lot of fat and less glucose.

One thing that helps a lot of people is to take some salt. There are a few people that are salt sensitive but it only amounts to less than 10% of the population. When going on a ketogenic diet fluid is lost and electrolytes can be lost when the fluid leaves. Replacing the salt helps. How important is salt?

If you have just recently started she might have what is called the keto flu. It is not really the flu but a loss of electrolytes. https://www.youtube.com/results?search_query=dr+ken+berry+keto+flu

It took me probably six months before I did not have a few bouts of low energy. Not all the time but just occasionally. If a person had been running completely on carbs it takes the body a while to get used to utilizing mostly fat for fuel.

Also make sure you are eating enough fat if you do not have excess body fat. Eating meats that are too lean is not good because the energy is mostly in the fat. Protein in the lean portion of the meat is mostly a building block for muscles and organs. It is not a good energy source as it takes nearly as much energy to digest as it provides. Fat provides the fuel for energy, protein builds muscle and body functions. Try upping the fat in the diet if not needing to lose weight.

Not medical advice. Just what I would do and have done.

John