1-The favorability has to be higher to yours, so it can be somewhat low if it is at least higher than yours
2-Its Random, they go out with them quite randomly at the start of a new day, so for example you start the day and Fio goes with friend A, then you load the save before that and start the day again, there is a possibility that Fio will go with Friend B or neither of them and stay home, the same apply with all of them
well thats what i remenber how it was when i played it on JP so i can be wrong, but i think i am more or less correct