what are the negative effects of only drinking diet pepsi during the day?

Only eating one decent meal a day will cause her to retain that meal as mostly fat, in preperation for her fasting throughout the day.

Sumo wrestlers eat one or two very large meals a day to get the same effect of fat/mass retation.

The general lack of nutrients will have symptoms similar to starvation - skin condition will get worse, nervous system will suffer (problems sleeping for example, nervous disposition, lack of sleep quality), immune system will dip (more colds/suffer more from them when they do happen).

Any time a normal diet is resumed, her metabolism will be used to a low calorie diet (I'd guestimate she's at about 650-750 calories a day, if that) and any addition of calories will result in weight gain. This is in contrast to a person who manages to have her frame/build but eating 2000 calories, a healthy active person can eat 3000 calories and remain low in body fat.

All that said, some people manage to pull it off quite reasonably - they will be hungry and they will have poor energy and concentration levels, they may even get poor sleep, but if genetics are on their side, the human body is great at adapting and coping with stresses. If the evening meal has some greens etc she may not do too badly.

Yeah, sad but true. non-diet pepsi is bed for you in so many ways. First of all the carbonated water disguises just how much sugar is loaded into each glass. this means lots of calories and lots of damage to your teeth. Also carbonated water is bad for you, they say it keeps fat deposits in your body an can cause cellulite (in women mainly). Even diet soda is bad for your teeth and has carginogenics in the sugar substitute. A better drink is fruit juice mixed with sparkling spring water

1 - I think you mean she "eats" an okay amount of food at night

Not good. ANY kind of pop is empty calories and high fructose corn syrup, which is worse than sugar. And she is probably cutting herself out on ALL nutrients and calories, besides the pop which is nutrient EMPTY!