Well poo...
At this time there is no way to import from Quickbooks, and there are no plans to add that functionality. The Quickbooks QBW data format is a proprietary, non-documented file format. So until someone documents the file format or donates a QBW file parser your best bet for importing your QB data into GnuCash would be to output your data in a CSV format and either import the CSV data directly or convert the CSV to QIF and use the QIF importer. -GnuCash FAQ
So much for hoping I was wrong... Intuit really is Pure Rapacious Evil.
But that does at least leave me with an answer (in fairness, no is an answer), so thank you paradisusvic! I will recommend they use GnuCash for their personal/home stuff going forward. The business stuff will require a different approach.
Client has been in business for 30 years, so their QB db is quite large, and a bit involved. Numerous accounts, add-ons, payroll, and loads of accounting stuff I frankly don't even want to understand. So my advice to them at this point, since 'Plan A' was to throw tons of cash at the problem to maintain the licensing for a what if something happens scenario. Is to make 'Plan B' a basically free what if nothing happens solution. Because no access to the data going forward is necessary ... Unless something (gets audited) happens - So...
Let the license laps, and put the QB Company files in "cold storage"
If something happens, grab a new subscription, pop the thing back open and do whatever.
If nothing happens, wait X years and then delete the damn thing.
It ain't pretty, but it ain't thousands of dollars - wasted... - either.