SL Ronith Blario - "...if you are in the attackers firing arc..."

As Firing Arcs are defined in the RR as any arcs on the pilot card and any turret arcs.

If an enemy is attacking Rey with a HLC, and Ronith isnt in the bullseye for the HLC, but is in one of the enemy ships printed attack arcs, does Escort Fighter trigger?

Yes, Escort Fighter can be triggered if Ronith Blario is inside any of the attacker's firing arcs, even if it's outside the arc used by the attack.