Well, I eat only once a day, (usually), and have done for many yrs.
I do it to keep my weight down and am now completely used to it and rarely feel hungry before late in the evening, tho thru the day I drink quite a lot of tea/coffee (milk, no sugar) as this keeps me going and well hydrated.
IMO, "daytime fasting" it's a good/natural method of LOSING weight and/or of AVOIDING weight gain. (Tho it maybe won't suit everyone.)
Why do we even think we should be eating 3 meals a day?
Eating at regular intervals/set times must be a relatively new tradition and associated with refuelling due to working physically v hard/being very cold?
In the UK now, many of us are rather sedentary, plus we have CH and warm clothes/shoes to out the cold. So we probably need to eat a lot less than our forbears did??
While our ancient ancestors, (I imagine), would have had very irregular meals, (when food was scarce), and would also have had to survive cold times without heating.
So surely most of us (not under-weight) adults are fully equipped to last till evening without eating, esp if we eat daily, drink lots during daytime & are in reasonably good health? (Nb Obviously, people with diabetes, stomach ulcers or in poor health should always eat small amounts regularly.)
Therefore, I think you should encourage & support your friend in her efforts to lose weight in the way she has chosen.
Maybe just suggest that she drinks more (non-sugary) fluids (eg tea/coffee) if she feels faint/nauseous during the day?
And/or suggest she could eat a limited amount of fruit/veg if she's starving? (Eg an apple and/or a handful of cherry tomatoes in the day wouldn't harm her efforts I think?)
It's important for her to make sure she's getting all the right vits and nutrients so any daytime snacks should at least be HEALTHY ones!
But if she wants to fast until evening, why not just let her try? (I predict that she'll either get used to it soon enough ... or else decide it's not for her and try another way!)
Anyway, wishing your friend every success re her weight-loss goal!