Good morning, Simon!
Thank you so much for providing these key details. So, on the face of it, yes, Google has likely acted with suspension because the two listings share the same general category, address and phone number. They likely believe this is a single business attempting to create spam listings for its different services, which is not allowed. In other words, they think your client is one business which offers both flat and thatched roofs, and while the roofing types are quite different, the business has signaled to Google that it really may just be one entity offering a variety of roofing.
The only surefire way to resolve this would be for the company to operate out of separate staffed offices with separate phone numbers, as well as separate websites. I would not be confident that you can convince Google to reconsider the suspension without this. This is actually a very interesting case, because roof thatching IS such a specialty business. Google could well have let this slide had the phone numbers been unique, but, given that the business didn't take this approach, they are in a pickle now.
So, your path at this point, as a marketer, is to see if the business has the resources to open a second, totally distinct business. If so, then you can start over again with the citation building process for them. If not, you may be better off telling them to combine the two websites into one, representing a single business with multiple roofing styles represented by a single Google Business Profile with multiple categories and good service menus.