Access to records during due diligence

(Posting anonymously to protect identity since I'm in a deal right now)

I'm under LOI on an eCommerce company and the seller is refusing to provide my QOE advisor with "reports only" access to Quickbooks and the Amazon seller account (70%+ sales through Amazon). Instead, the seller wants to pull reports and send them over in PDF format as requested.

My diligence advisor is about to back out of the engagement because he does not feel he can meet his professional standards without getting access to these two key systems. Has anyone encountered this? Would you be concerned by the seller's unwillingness to provide access? I'm getting uneasy since I really need to verify the sales, COGS, and advertising spend at the very least and it's hard to do this without "clean" access to at least one of these two systems.

For context, there is no third party accountant on the seller side- the seller has done all the accounting and filed all the tax returns without professional assistance and the seller is not trained in bookkeeping/accounting.