That's such a terrible argument. A person can't consent to being raped at a later date. If a girl says you can do whatever you want to her in ten minutes, she's still allowed to say no or stop at that time. A corpse can't say no or stop - for obvious reasons - so previous consent is not affirmed in the moment (because it cannot be). That's why necrophilia is always rape.

Of course, it doesn't even matter rust necrophilia is rape. What matters is that necrophilia is necrophilia. It is inherently and categorically wrong regardless.