Are you sure he didn’t scratch it when opening it? The scratches on the bevelled part of the caseback are 100% from a clumsy opening.
Anyway, it’s extremely strange to want to preserve a gunky mess of a caseback seal. Most collectors would want a thorough clean and service. At the most extreme they’d leave the scratches on the crystal but most would polish them out. Sounds like he either want to buy it to sell or doesn’t want you taking to someone to service it who doesn’t know what they’re doing and might decrease the value.