Myself don't think froglet swallowed sand. For most babies we recommend damp unprinted paper towels as substrate; but obviously with aquatic/semi-aquatic species we can forego that. If tail not resorbed into body; then nothing was eaten that could hurt it. And if water was available, it did not dry up.
Do you have a water analysis report on your well? Maybe you still want to use Seachem Prime on it to counteract any possible metals in it? Again, sorry for the little one, was so cute.