Seeing someone else tickle my wife has always been appealing to me no matter who it was, although I suppose if it were ever someone with ulterior motives I probably would have a problem with that.
My wife doesn't get turned on by being tickled and views it as childish, although she is very ticklish, and with me she has to be in the mood (which isn't very often.) She has always been a bit of a control freak, but in her more playful moods I have managed to tickle the heck out of her from time to time. A little alcohol has also helped on occassion, especially since I am much larger than she is and can hold my liquor better than she can. Over the years these "outside" tickling experiences have become less frequent. I still have those memories of her being tickled over the years, encompassing various scenarios, and by friends (male & female), family members, and even her young nephews when she used to babysit them years ago. Some of those instances were more than just a quick tickle and were most likely due to how she reacted to being tickled, which was a mixture of her hysterical laughing and pleading combined with her modest embarrassment at losing control the way she did. I have to say though that many of those experiences are burned into my memory and in retrospect are still far better than pratically any clip or video I have seen.