did bum grandmother set him up, or is it just me

I can't believe your friend think it's not a set-up.
It's obvious enough already, unless she/he is new to
this manga reading world I guess some might think no.

She's not a saint, definitely. She had more or less contributed to the abuse. But perhaps your friend's point is that we are not very certain if she 'knew' about the rape specifically. She seems to be okay with the uncle beating Bum, just that he should be cautios as to not 'kill' him. She seems to also be feeling guilt and try to compensate in other ways (i.e. sneaking Bum meals)

I also think she deliberately led Bum into the lion's den. Perhaps she knew something bad would happen, but canonically, there has been no mention that she knew about the rape itself beforehand.

She let it happen to save her own skin and comfort. Because if she called the cops and he got arrested she would no longer have her means of financial support. Well that's just how I saw it. It's not a big house. She was home wasn't she? So how did she not hear anything?

apart of me thinks that she did it for revenge, cause while she was getting her ass whooped by her son, bum was at the sauna relaxing (maybe she felt that bum betrayed her)

i'm not sure if she knows about the rape but i'm certain she knew something terrible was going to happen to bum if he went in that room so she's still scum of the earth if you ask me

i'm not sure why, but i'm hating the grandmother way more than i'm hating the uncle and what he did is so much worst right now

i just feel like she was suppose to protect sweet bum but instead she offer him up to the devil

poor bum, and he's still not getting the love he deserve

Yea, but she's the one who gave the sauna money though? But yea, she took a beating after so perhaps she had a lapse of judgement and turned Bum over to the uncle.

That old bitch did set him up. As soon as she said she felt like sleeping alone I was like awe hell naa this old ass lady on some other shit. God her and her fucked up son made me so angry. Bum is just so broken and its sad.

There is no evidence so you cannot simply accuse the granny. You can hate the idea but you shouldn't hate granny because you assume her to know Bum was being raped. No, that's not how it works. You assume on action but we are not a mind reader.