It's got to be something she thinks is very urgent, or she wouldn't have still been in her nightdress. Like she didn't want to keep him waiting for even a minute or two to throw on some clothes.
I can see it being something like this as a possible scenario:
-- Joy is in her dressing gown, it's quite late. Her flat's intercom goes, she is not expecting anyone but because she is a kind and trusting person or perhaps she has neighbours who sometimes ask for help or some other reason, she answers.
-- It's someone she is acquainted with, most likely from her Church group.
-- They present as troubled, they have a problem, they ask for her help or if she can listen. As you say, it must be compelling enough that she has to listen. Maybe this is someone who has expressed suicidal ideation to her or has expressed another, very difficult emotional problem and she feels compelled to listen. The man in the CCTV footage is older, about her age or a little older, so perhaps she does not see him as any threat whereas she might have considered a younger man a threat?
-- She buzzes the guy up even though it's late and she is getting ready for bed. She knows him so she is not afraid to let him into her home at this late hour. Has he been there before? How does he know where she lives? I wonder if he has been there before, as if he just showed up at her flat, knowing the flat number to buzz up to (no one else in the block said they let him in), that would be super creepy and frightening and she prob would not have let him up.
-- He comes into the flat. She makes 2 cups of coffee, one for herself and one for him. That suggests the plan was to talk something through.
-- Either he planned to rape and kill her and that's why he went there, or he didn't plan to, but thought that Joy letting him in, late at night, when she is in her dressing gown, is a "signal" that she is interested in him. He tries it on, she rebuffs him probably quite forcefully and he turns violent and assaults her, then in his rage kills her so she can't identify him, then panics and tries to destroy evidence. If he had been there before perhaps he thought he was starting a relationship with Joy.
I can't see a scenario where she didn't know him, and I do think she must have known him well enough to think it reasonable to admit him at that late hour. He had her address!