I think it's safe to assume that the previous body of water was continually "topped up" from natural spring(s) underground.
These springs now have nowhere to go. Water will take the easy route, in this case, into his property.
Embedded in property law is the principle that no property should ever cause "nuisance" to anything adjoining. This includes discharge of surface water/rainwater etc. Or, in this case, diversion of watercourses to the detriment of neighbours.
This is very difficult for lawyers to prove. There would have to be evidence that the offending water is "new".
Usually, Building Inspectors should take this into account when inspecting the adjacent works.
Also, Environmental Health should be made aware of this. They will need stirring, and that might prove sluggish. It still remains though, that damp or standing water is properly regarded as a potential health hazard.
I think I would start there.
Although pumps etc are a pragmatic remedy, they only tackle the symptoms... not the causes.
If you've explored everything with the local authorities and they prove ineffective, then I would take the pragmatic approach and deal with the water/damp myself.