the situation: 2 characters are flanking an enemy, and one player holds an action to cast Immovable Object on a net if an enemy is entangled in a net, and then player 2 uses an action to attack with a net and tangle the enemy up. assuming the second player hits with the net, what would be required of the caster? would they have to make a spell attack etc. if the player successfully casts the spell on the net and the enemy is trapped in an immoveable object net, what options would they have to escape?
It's up to the DM.
There are a lot of moving (unmoving?) pieces to this interaction that are not entirely clear. The DM is going to have to rule on something somewhere.
Immovable object doesn't protect the net from swords.
The rules for being trapped in a net state:
Dealing 5 slashing damage to the net (AC 10) also frees the creature without harming it, ending the effect and destroying the net.
Nothing in the spell description of immovable object can mitigate this. So the immovable net can be destroyed by dealing 5 slashing damage to it.
What happens next is unclear.
Did the slashing damage destroy the net enough such that even an unmovable net is not enough to contain the target? Who's to say? (The DM is to say).
Trying to break free from the net using strength and move the net using strength is glitchy, DM may need to patch.
The spell is somewhat glitchy when used with a net. Here's why. The net says:
A creature can use its action to make a DC 10 Strength check, freeing itself or another creature within its reach on a success.
Immovable object says:
a creature can use an action to make a Strength check against your spell save DC.
Arguably, you have to be able to move the net to attempt to escape from it, and attempting to move the net requires an action, but so does escaping. Technically, it would require two actions on a single turn to try to escape from the net using strength. This makes it impossible to escape from this way, technically.
In this situation, I would allow the target to attempt to escape using a single action, making a strength check against the appropriate DC for immovable object, but at disadvantage because it is a net.