Anybody, husband or wife have experience with Cuckoldry?

I ask, because, while I am married myself, this isnt about me. My best friend has confided in me that it's her husbands greatest fantasy for the past 10 years, but they've never done anything about it.

Its his birthday on Christmas eve, and she is considering making the fantasy a reality, as a birthday gift.

She is considering either her ex, or one of her husbands friends to be the guy she has sex with. She asked my advice, I honestly have no clue how to deal with it.
Not because I find it objectionable ( I don't ) and not because I am on some moral high horse or suffer from social conditioning that immediately says Dont do it!!

They are adults, very open minded and have a great bond, very honest with each other. There are other factors too, but mainly, what do people think?

Any experience of it, any advice on planning it, etc?

(im not interested in people who immediately think she shouldnt do it, because as an adult, she has made her mind up to do it and I support her, I am also fascinated)

I am just interested in how people think she can make the most of the event I guess.

Thanks!