Suspicious behavior:
Sorry man, but yea, you posted your thoughs because you feel like she either has or will cheat on you and you're probably right. Her behavior is suspicious and her story about why she took off her ring sounds like a lie. There is no way to know for sure, short of a confession, unless you catch her in the act.
The next question is what is the condition of your marriage now? Underlying problems in relationships can manifest themselves, for some people, in the form of cheating. Some people cheat because there are unmet needs in their relationship. Until you both understand the dynamics of what is behind it all you won't ever get past this.
O' and the "friend" who encourages her to double date (cheat) on her husband is bad news! I assume this double date did not include you? If she is feeling confused about things and is not happy then hanging out with someone who feels it's ok to cheat and encourages her is just a recipe for disaster.