They’ll definitely be working with the rebels beyond the rescue! He’s getting busted out by a disillusioned veteran of the army that captured him, and that guy is already geared up with a battered and customised mech stolen from that army. The other player characters are going to drop down and fight to rendezvous with the rebel and their friend, at which point they can drop a third mech prepared in advance for that player to join the fight. The rebel leader will definitely stay relevant, he’ll just have fewer forces to back the players up than he otherwise would have

Lancer has a system called “reserves”, which are basically special bonuses you can acquire by either achieving extra objective in a fight or securing something in the downtime between fights. Stuff like being able to call in an artillery barrage from allied forces, or a flight system with fuel for one fight. These are usually things only for the players, but I’ll be treating the loss of rebel forces incurred in rescuing the player as a reserve for the enemy — the rebels aren’t able to garrison the town they captured and it’s not longer a safe spot to rest, or they can’t take out the artillery spotters before a fight, something like that depending on what’s relevant and how they decide to approach things.