If ever there was a lesson in working via a Ltd Co. it’s this, but I guess the OP already knows that.
When I worked in a bank it was pretty easy to trace people via their credit records, but that not an avenue open to you or any legit PI provider. I’m guess they’re going to trawling the voter rolls.
Before you decide to go ahead, it’s worth considering, how likely are they going to be to be able to pay their share? If they are, will they? Do you have a contact or the like which proves them liable? Are you willing to spend thousands to trace and then enforce it?
My guess is that they’re probably not going to be in a better position than your Dad, I guess they’ll also be of retirement age themselves now? Are they going to want to hand over big lumps of their retirement fund for a 18 year old debt that’s likely to be completely unenforceable?