Unless downtime of your application results in money lost for them (as opposed to bad decision making) then I would probably say no to this. If it comes down to

Unless downtime of your application results in money lost for them (as opposed to bad decision making) then I would probably say no to this. If it comes down to "Is the data science good enough?" that is basically a sales objection. If you can, talk about success with other companies and your track records. There are legitimate reasons for a customer to need to see your source code, verifying that you can do your job is not one of them. Treat like a sales objection.