I don't want to get anyone upset, but the whole thing sounds really creepy. Any chance your friend, her husband and your husband are interested in wife-swapping and are testing you out to see what your feelings are on this? Wife swapping is not an uncommon occurance amongst Witnesses. I remember it happening in my area 25 years ago and it still goes on.
The fact that your friend's husband told you he "found it hot" watching his wife with another man says volumes. Most men would flip out if they caught their wives fooling around and would beat the living tar out of the guy.
I know of two married Witness couples who hung out together all the time. They'd start drinking and talk about their sexual fantasies. The one elder openly said that he had dreams about the other guys' wife and she said she fantasized about him too. The elder's wife told this woman's husband that she fantasized about him. He did not feel the same way and felt very uneasy about these discussions. Ironically, he's the only one who left the Borg----the other three are still all Dubs "in good standing."
On reflection, the guy who left feels that they were probably all testing the waters to see if anyone or everyone was interested in the ol' wife swapping. He most definitely was not and was disgusted that people who called themselves Christians, would do something like this.
Christ, with friends like this, who needs enemies?