Is SAM applicable in this case? The company has no contract with either in respect of the supplied modules (other than the contract to deliver the final product). The company has no ability to manage the customer as they would a 3rd party supplier.
Wow, this reminds me of the time I met a judge and asked him "I have this friend . . . !"
It's a really interesting question. We often debate the merit of "alternative practices" amongst the SEI community (read: process nerds) with MOST people insisting that their "alternative implementaion" of SPs are "alternative practices." Usually they are not. Alternative practices are an accepted, albeit rare, way to satisfy the Goals in CMMI, which I'm sure you know are required.
In this case, I'd say "we've got a live one here maw!"
Most of the practices in SAM SG1 are about selecting and acquiring a service/product from a supplier. Even though you're achieving SG1 your customer situation may very well serve as an alternative for the practices within it. Without knowing all of the details, I'd say there is a good chance you'd be OK here.
It's not so clean for SG2. You do, in fact, accept the product, and I'm sure you make sure that it works correctly, and that it can be integrated into your environment. So you're not completely off the hook for SAM.
Whatever you decide, I suggest you document your situation and when you're ready to engage a Lead Appraiser (hey wait a minute . . . isn't THAT SAM???) you discuss it with him/her to make sure you see eye-to-eye on this. If you don't, get a second opinion.