How sad for your friend.**:(* I've not grown these, but many plants have the same requirements and I've grown plants similar in requirements to this one.
"There are some green leaves left, so it is not dead, but maybe on its way? Do you think it will survive after being so wet for so long?"
It's difficult to say.* Sometimes root rot sets in and doesn't even show until the plant is watered a couple of times.
"Should they wait longer to water than 3 days since it was so drenched?"
If the new soil is dry to almost dry, I'd say to water after 3 days as instructed.
"How often afterwards should it be watered? I told him NOT to water everyday at all."
From this site which has lots more growing info:
"Keep the soil moist in the summer, but allow the pot to dry out between waterings when indoors. These plants are heavy feeders, so fertilize every 2 weeks throughout the summer, and monthly in the winter with a water-soluble fertilizer (if it's planted in soilless medium; it may require less fertilizer if in real soil)."
So I'd say to let the soil dry out between waterings.* Then let the water run through the drainage holes. Let the plant sit in water for about 5 minutes so all the soil gets wet, then dump any excess water.
"Will all those loose roots be useful to plant after they have dried out, and will it grow into another plant?"
I don't think so without the crown of the plant.
"Can I keep it outside during the summer time?"
Yes, but don't put it directly in full sun right away.* Let it get used to the sun first.* This site has helpful info on growing them too.
That was nice of her to give you one.* I hope they will all be ok.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.