It can be saved. Mine have healed from areas worse than that. But you have to correct whatever caused the decline. What are your water perimeters? It does not look to be from light causing the decline. But it should be in med to low.
The decline area has algae on it. Your phosphates may be off as well. If the algae is not controlled it will make the favia healing very hampered.
My parameter are all in check
Nitrate---0
Nitrite---0
Ammo--0
Po4--.o4
Kh--8
Calc---480
Spec.grav. 1.024-1.025
Mg 1400
Ph. ---8.1 .
Everything else in my tank is thriving ! I didn't have it near any other corals. But I did cut a mojano out of the back of it. I had it on the bottom and just moved my pavona higher and I placed the war coral in the pavonas old place.
Your numbers are not bad. Hopefully it will heal itself. The algae growing on the skeleton will stop the healing and it will not heal over that are with live algae. I would try and move it to an even lower light area. Maybe something that has some shade. A pavona and favia prefer to same light levels, except a pavona cactus they prefer high light. It may have gotten burned and the algae grew on the exposed portion. The lower light area may also help rid that area of algae.