Anne- I too have struggled with this question for some time.
And- I can't say that I have any LARGE amount of real world practical experience with it (although I have done it once- both ways), but...
What I would say is this:
...Do *YOU* like her?? ...Are you curious about what it would be like to kiss or caress another woman?? ...To have you and your husband touch her? ...Are you and your husband both comfortable enough (and mature enough) with each other to explore this in a mature, loving (and hopefully, fun!!!) way?
If so- then really, I don't think anything at all should stop the two of you from exploring these desires together.
Although- after reading what you've said, it might be good to re-think your choice of person. ...Maybe try someone less close to home?? ...Someone who won't get complicated by her husband??
Now again- often times, I think some men may come to their wives and ask for this because they secretly want to just be with the other woman. But, if this is the case, I think you, (or rather he) is approaching it all wrong.
What you need to understand here is that its all about sharing. ...And about adding a little bit of occasional extra spice to this long relationship that you both have been having for most of your lives.
Again- not to sound sexist or whatever, but I think if its done the right way (and with the right rules, which you both agree to in advance, and not *everytime* all-the-time), it can add quite a bit of flair to a now-gone-routine sex life.
Also- I think its important that she needs to understand that you love your husband, and that she is there for both of you- and it needs to be very clear that if the two fo you start getting into dark water, its time for both of you to permanently break it off with her.
...Which is why I think its a great idea to try it for the first time with someone who is clean, who you are both comfortable with, and with whom neither of you will be likely to meet up with again.
...When your husband realizes that you are as much into it as he is (oh- and, *shhhhhh*- that you secretly control exactly when, exactly where, and exactly how it happens), he will turn over backwards to keep the fire of your marriage burning.
After all- when your wife fulfills all of your wildest desires, what better fate is there for a man to have??