A creature, other than the one restrained by the net, can make an attack against the net (AC 10) which by dealing at least 5 slashing damage will free the restrained creature.
If a player restrained by a net - who is adjacent to another player - is grappled by some creature and carried away, could the adjacent player make an attack of opportunity against the net, thus ending the restrained condition?
You can't make an attack of opportunity against the net, only the creature provoking the opportunity attack. From the rules under Opportunity Attacks from the Basic Rules (emphasis mine):
You can make an opportunity attack when a hostile creature that you can see moves out of your reach. To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack occurs right before the creature leaves your reach.
The net is an object, not a creature and cannot provoke attacks of opportunity as objects do not provoke opportunity attacks. Therefore you cannot target it in this way.
Additionally, a creature being dragged out of reach does not provoke attacks of opportunity. See Can I grapple and drag someone to grant opportunity attacks against them? for more information.
From the Combat chapter of the PHB (p. 195; emphasis mine):
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
Since a net is an object, not a creature, it does not provoke opportunity attacks.
