Dood...you picked a BAD choice of tankmates for a lionfish (if your LFS said it was OK, it's time to find a new one). The chances are high that it was the triggerfish, esp considering that the exophthalmus is unilateral, which suggests the cause is injury-related. In nature, as well as aquaria, triggerfish will typically kill lionfish outright, or at the very least, harrass them to death.
As for the lion's eye, it's probably either completely gone or it will be sightless. You need to get the fish into a stress-free environment, which means moving the lion or removing the trigger. It would probably be best to move the lion since you may need to treat the eye with a broad-spectrum antibiotic (Furan 2 is the current one of choice). Keep the lighting low, and the water clean. It might not hurt to run the tank on the cooler side for a bit to prevent or slow any possible bacterial involvement. 74*F max will be OK for this.
HTH
