Honestly, that’s all you can do. Rebuild as best you can, document any estimates and disclose. What’s the alternative? I suspect you will probably be very scrupulous with the paperwork so that will go a long way in assisting your client.
As for W-2s, I’ve been able to get those for an individual. It doesn’t contain information for the state but you might be able to get what was filed for the employees W-2s/W-3s. Here is something I found that might help: https://secure.ssa.gov/poms.nsf/lnx/0102019001