How can you turn robust excel VBA macros into a product?
Once you build out the Excel plugin, you can simply sell the software via the web. Set it at a price point that's going to be less than what an average accountant needs to get approval for (typically $500 is the max, but it varies by company). This way they can just buy it and expense it, and not have to worry about it. The last thing in the world you want to get into is trying to (a) sell into the Big 4 or (b) sell into enterprise accounts. Both are going to be more trouble than it's worth.
Also, make sure your co-worker won't get into legal trouble by selling this. That IP may be owned by your company, especially since he probably uses it for his job.
It might be difficult to sell to them though, as those machines are locked down tight and the accountants can install nothing without IT intervention. IT is usually controlled by a central group that runs IT for everyone (as in, all the member countries), so it's gonna be difficult to get an exception for your product, as they have internal tools developed that fit in with their other custom Audit Workflow Applications.
So avoid Big 4, but try selling to 'average accountants' as endersshadow says, who might work in a less locked-down environment.