How long can you live on just apples?

For a good while. Why would anyone want to? You can eat a well balanced, healthy diet and get everything your body needs. Apples are fine as a snack, but they don't provide nearly the total nutrition a body needs daily, which means the body would start seeking that out from within itself (muscles, bones, organs, etc).

Your friend is wasting her time, unless she lives in a remote area, has no money, and apples are ALL that will be available. Otherwise, to limit oneself to just 1 food is stupid and produces no benefit. If she's wanting to lose fat, she can do that the healthy way with much better and longer lasting results.

5 days is not good.

Perhaps if it was just for a day or two, but not five.

She will be irritable, tired and low in energy by the end of day 2.

they'll just end up malnourished

ok