sorry to hear she is discouraged.
there are die-off points where some fry may fail to develop properly and die, but itll be a handful (if any) over 48 hrs, and then no deaths after that. some new every day, means something is hurting them.
but thats nature. in nature, out of all the spawns a betta in a stable population has, only 2 fry (on average) would survive to reproduce. so tell her not to feel too bad.
how much water can the shoebox hold? ive heard of spawns in such containers, but i prefer 10 or 20 gallon tanks filled partway, then filled up over the first 2 weeks. more water is better than less. but different methods can achieve similar results.
so there is sometimes NH3 above 0ppm? thats likely your problem.
the tank needs to be cycled. do you have other tanks? take a small handful of biomedia from them and put it in with the fry.
ive never spawned without any filtration. hell once i even spawned with a HOB filter on low with a current killer and sponge over the intake (it worked too). i like sponge filters though. set it on 1-3 bps during the spawn and first week, then start bumping it up. my mustard gas fry are 25 days old, and their sponge filter is on at 75% strength. im also changing a gallon a day each day this week, and will do 5 gal a day next week, the week after ill be doing 100% (10gal) WCs daily. clean water is the best thing there is for em. just be sure its the same temp and params.
doesnt matter how you heat the water. if its stable at 84, temp is not the problem.
at 2.5 weeks, i say start up those BBS asap. as i said, i know where she is in regard to the trepidation in doing it. but it is surprisingly easy. the little shrimp can hatch in just about any mixture. then strain some through a coffee filter, rinse with tap, and swish em into the tank.
here is a great resource on betta breeding, helped me out a lot when i was getting started: http://bettysplendens.com/articles/c....imp?catid=856