It sounds like your friend is quite free with offering these services. I would guess that it probably just isn't a big deal to her and that's why it didn't come up in conversation with you. Since nothing is happening between your OSO and this friend of yours I think I would just let it go if I were you. It sounds to me like this is her way of letting people know that she finds them attractive.
It is your OSO's responsibility to let you know what is going on for him. It sounds like he's done this and has been reassuring to you that there is no relationship between him and your friend. There's always the option of talking to your friend and letting her know that you would like to know from her if she is interested in one of your partners before approaching them. If you have expectations of someone it's best to let them know what those expectations are. If you do tell her and she does it again then you have cause to be upset about it.
Everything will be ok in the end. If it's not ok it's not the end.