Myself would do the Pedialyte or similar unflavored fluid electrolyte bath first. You can use the rainwater for all the frogs needs. Any other water sources (tap, well, bottled spring) will need to be treated with a dechlorinator like Seachem Prime or ExoTerra Aquatize.

OK, on to bath! Get a small critter keeper or a plastic cup with holed up top and fill with solution of 90% water (80F) and 10% electrolyte to frogs chin depth. Place frog in it (wait until frog is active at night); place top, and cover with towel so frog relaxes for 20 minutes. Afterward you can take water solution out and try feeding frog. If weak; imagine will have a better chance of catching a cricket in small container. Crickets should be same or smaller than distance between frog's eyes. If frog eats, you can release and it should be OK .