It’s actually possible to have water retention with low sodium. The body will remove water from the blood into the tissues in order to increase blood sodium concentration.
All of her symptoms sound exactly like low sodium.
When you supplemented sodium more, was there also more total fluid intake? That would continue to dilute blood sodium if even though you’re getting more salt you’re also getting more water...
I would supplement salt without increasing fluids and keep at it for at least 4 days. Aim for 5000mg sodium a day. That’s about 2tsp salt plus salting foods.
If you hadn’t mentioned the swelling, my other questions would be how much is she eating overall? I would’ve been wondering if the problem was being driven by too little intake altogether. But that doesn’t explain the water retention like low sodium does.
Still possibly a factor though?

Sounds like her calories are kinda low. Also adults need 3500-4700 mg of potassium per day which is hard to get on this woe. I was exhausted and found any exercise difficult until I started taking a 1/4 tsp of No Salt (Potassium) every morning w a tsp of Natural Calm (magnesium). If I take more I get diarrhea

Low calcium can also make it hard to retain enough sodium/magnesium/potassium, regardless of intake.

You do not need calcium supplements 99.99% of the time. Most of us don't get enough D3 - and if we do get D3, we don't get enough K2 for our body to route it through.

D3 helps soak up calcium, K2 is the bus driver to get it where it needs to go, magnesium and potassium are the GPS for directions and the passport to get the nutrients into cells, and boron and zinc are the bouncers to keep those nutrients locked into the cells - so dehydration won't leech all the nutrients from your bones and muscles - causing cramps and/or bone pain, increased injuries, weakened bones, etc. In extreme cases, the nutrients can be metabolized (to save your life), but you'll get the signs of dehydration quicker when your body isn't stealing all the resources to keep that most delicate electrolyte balance in your blood.

Most people get enough potassium naturally when eating whole foods, but magnesium is generally required to be supplemented. Potassium isn't required (currently - it's in process) to be tracked on all nutritional labels, so people often get more than they realize. Too much potassium as as deadly a risk as too little. I personally recommend people speak with a medical professional before adding any potassium.

There are a number of factors here. I'd test drive consistent electrolytes for a while. A little late evening swelling while the body adjusts to new things isn't mission critical unless there are other factors. Adding a little fresh lemon into water can help pull some of that swelling back out...if it's just water retention. Thyroid imbalances can also cause fluid retention, as can undiagnosed diabetes. If this continues on, I would see an endocrinologist or someone else fully versed in the benefits of a nutritional ketosis dietary plan. There are many functional medicine doctors who now do online Skype type chat consults and such, too.