At first, your friend should've quarantined them. Disease can easily spread with new additions. There should also be at least 4-5 females as two females might gang up on the third.
Mollies prefer brackish waters; and they'll be weaker in fresh. They'll live longer with a little marine salt in the water; which the other fish can't tolerate.
Once he's done finishing all that, ^ He should introduce the females after he has quarantined and acclimated. :D
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