What are the consequences of hail damage to a small GA plane?
Like anything in aviation, it depends... 

There are a lot of discussions on the internet about it you can find 

Here (mooney) 

Here (cessna)

And Here 

It is a case by case situation and depends on the extent of the hail damage and what/where the damage occurred. In many cases the issue crops up that you can not simply bang the dents out you would need to reskin the panel. This can, depending on the aircraft be a costly process. 

There are planes out there flying with light hail damage and it causes no problems, in many cases its purely cosmetic. A lot of planes flying out there have other similar damage (tool drops, bird strikes, impacts from rocks etc) and fly just fine. 

An good A&amp;P would most likely be able to tell you if the particular Cessna in question is still airworthy.