Firstly, you'll want to look into Visual Studio so that you can build a plugin. Secondarily, you'll also want to do some market research. The Big 4 all have internal tools that they use, anyway, so I'm not sure that's your target market. Your market will probably be SMB's that are doing everything in Excel. Some enterprises do a lot of accounting in Excel, too, but almost universally, the heavy lifting is done in a robust ERP like SAP or whatnot.

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.