If its built in 1900 then its unlikely to have a DPC. Whats it got to do with DPC anyway? Downpipe leaked – damp not rising damp. Render wont have anything to do with it unless there is driven rain, most likely damaged by the leaking downpipe.
I dont know scottish law, but as a general rule in these circumstances external/fabric REPAIRS are join responsibility. Adding a DPC where none exists is IMPROVEMENT, so not join resp. You are only liable for internal damage where you have a duty or responsibility for maintenance of the fabric and have failed in that (as distinct from just a liability to pay for such maintenance). I cant see any positive responsibility there unless scottish law provides.
In any event, he must have noticed it as much as anyone else and did nothing, you are not liable for his inaction.
I suggest ‘see you in court’