My best friends boyfriend wants to sleep with me

My best friend (18F) and I (18F) have known each other for six years and she's been with her bf (19M) for about 2 years. We haven't really been best friends until a situation this year but that's unimportant, just explains why I didn't know this. Tonight he texted me out of the blue and starts talking about how they have an open relationship and wanted to know if I would take him up on his offer. I said no for a thousand reasons: he's my best friends boyfriend, I'm dating someone, it's just a big no and I've never been attracted to him and was happy with him as a friend. Even tho he said both my best friend and him are aware of the open relationship he doesn't want me to tell her. I don't know what to do, tell my best friend? The guy I'm dating? One of my best friends and Is mutual friends? It seems wrong to keep this secret but then again if it's an open relationship is this ok? Please help.

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Update edit:

I first asked my best friend if A) she truly was in a poly relationship and B) if this was ok with her. Turns out they are polygamous and he had asked her permission if he could so it with me. I then told the guy I'm dating and he told me my best friends boyfriend had also asked his permission before asking me. talked to a mutual friend about it who's been in a relationship for 1 year and he'd apparently also asked her and her partner if they'd do a threescore. still seems sketch to me but I've moved on. thanks for the advice!