so the humour relies on knowing about two humour/banter tropes: 1. "your mom" type jokes where one person makes some lewd or insulting suggestion about another person's mother (e.g. that Jimmy's mother wants to have sex with Dude). 2. The subversion of said joke where it is revealed that the mother in question is dead, followed by the instigator implying that having sex with a dead person is what they wanted all along. (e.g. Dude saying "I know")
This second trope is further subverted by saying that the dead mother in question actually consents and is in contact with the instigator.