Because of the size of the lesion I wouldn't think it was a bite though it certainly looks like one. I don't think an Angel would be able to take such a large chunk out of the fish, though I could be wrong. If it was an angel bite I would expect more tattered fins and a lot more signs of some sort of conflict. There are other possibilities to consider:
- Mechanical damage, fish got caught, panicked and pull its way past something sharp or abrasive. (could be the result of interaction with the Angel)
- Burn, I had a fish rest against an older heater so long that they burned their skin, it started with discoloration but then turned into a large wound. (I had to move the heater)
- It could be the Dwarf Gourami disease, I have no direct experience with this disease.
I would follow the advice of others, more water changes, clean clean water. Observe the fishes behaviour and watch for any improvement. It might be prudent to put the fish in a hospital tank if you have one but likely if it is the dwarf gourami disease it may have already spread.