We do not keep the books of general contractors. Not yours, and not the GC your sub works for on his other jobs. That is a deliberate limit on who we take as a client rather than a rule about who inside the firm may look at what, and it exists so that the question your sub is going to ask has a one-sentence answer.
That answer is simply that his customer is not our client. There is no internal division of duties for him to trust, no wall he has to believe we maintain, and nothing anyone here has to choose not to look at. If you would rather have the mechanics as well: access in QuickBooks Online and Xero is granted per company file, and we are only ever in the file of the business that hired us.
It runs the other way too, and this is the part worth being blunt about. Introducing a sub to us does not give you a window into him. We do not report his financials, his margins, his job costs or his bid detail to you, we do not monitor him on your behalf, and we will not answer questions about him. What you get back is a sub whose paperwork keeps up with your job. What you do not get is his numbers.
Where the relationship exists we tell both parties it exists, and the same commitment is written into the engagement agreement. If you would rather we did not take a particular sub at all, say so and we will not.