Big spoiler, but it's worth your time if you want to know about Inhye-noona.
In fact, I just used Google Lens to read the chapter, it seems like Inhye was employed 4 years prior to the start of the story, but quit her job because she couldn't take the workplace bullying going on (aka, seniors dumping all their work on her). A little more context on this, she is doing another overnighter when she receives various notifications telling her "Happy Birthday!" from the girls shown in this chapter. This is when she figures out that she's being overworked, quits, and basically becomes a NEET. Inhye lives a few (?) blocks away from the sharehouse. Considering that she was masturbating when MC saw her, I think it's highly unlikely that she hooked up with MC's grandfather. Amen.