so sunshine...what did you decide to do?
be careful, she may be "testing" you and to see how much you value your friendship and/or she may have suspicions about you. do they "swing?" if not, she may be wondering if the 2 of you (you and her husband)are attracted to each other. if you say yes that you will do it, then there is her answer.
i agree that you should not do it. nothing good will ever come out of this. (with the exception of the sex at the time). besides, try to picture yourself having sex with your friends husband? with her watching!! not to mention his penis has been in her vagina and you could feel comfortable with that? do you honestly think if you go through with it that it'd be ONE time? NO! what if the two of you develop feelings?
did your friend already ask her husband if he'd be willing? or did she ask you first?
waaay too many angles to this situation and i see nothing positive. you play with fire and you will get burned.
there is a frequent poster here will offer a different side to the coin here. if she sees it, she will respond.
Is it normal? Well, that depends on who you ask. I would find myself pleading temporary insanity if I found my friend in bed with my dh. If you consider her a "good friend," I don't think you should do it. You and/or he could get emotionally attached (or she might just think you are), and the friendship with her could be over. Chances are, they'll want to continue to include you in their sex life. Would you be okay with that? It just sounds like something that will eventually get very messy, but ultimately, it's up to the 3 of you.