Fish generally re-grow fins very well. The bigger issue would be the fact that the fish in the tank are biting the angel's fins off. That could be an on-going problem since angels have such tempting fins.
Honestly though, even if the fish lost its tail to a nip, it would likely still be able to move around better than you are describing. Not necessarily, but likely. It is possible that your fish actually has a bad case of fin rot that popped up very suddenly.
Clean water is always the best start to curing any illness. I suggest you start by doing a large water change, at least 50%.
Next, does tail stub have any white or black around the edges? Any general lethargy? Or does it seem like just a bit of on issue swimming from the tail loss?