They don't know how to shoot a wedding and want to be a second shooter. Not an assistant. A shooter. And they just decided to start a photography business. So I suppose they might expect to get paid. I think most second shooters do, even if minimally. But based on what body of work, particularly dealing with photographing people in a unique event and under a significant time constraint. And being a brand new business owner (maybe) are they going to be able to stick to the schedule the primary has to follow, or will they be faffing off to shoot their own gigs, or work whatever other job they may still hold. And of course, they apparently don't already know any wedding photographers that would work with them. And they don't know what info to offer up in such a request to make it seem feasible. Plus, what happens when it comes to owning, editing, and sharing the photos, do they know yet how to navigate that, since it directly impacts the other photographer.
The issue is the powerful indication that this person is asking to bite off too much, and at the potential detriment of the photographer who allows them to do it at one of their booked weddings. With everything else involved in shooting a wedding and delivering final images, will a photographer entertain a request presented in such a way.