I agree with the others that say that completely open, honest communication is a great idea at this point. Use all the appropriate words for what you are talking about, with no dancing around the topic- if you're talking about swinging, directly say the word "swinging"! But in my opinion, this should be just one topic of a wide-ranging discussion that encompasses the entire topic of your wife's happiness and unhappiness.
I don't want to hit this too hard, but I thought I saw a couple of little signs that might suggest that your wife might not feel free to express herself as an equal partner in such a conversation. You mentioned that you knew she had stopped her communication with a guy previously, but did not want to go into your basis for knowing this. This suggests that either you are sneaking into her personal electronics, or made separate contact with the guy. Both of these suggest a lack of trust and a lack of openness that, if your wife senses it, might inhibit her from feeling safe to be be open and honest with you. Also, you mentioned that, in your discussion that was kind of about swinging, you laid out a series of ground rules to her. Besides being a very premature topic, this seems like a situation that is best approached as an open discussion and agreement between equals, rather than one in which you lay out the rules to her. In an atmosphere like that, she may not feel free to express her own desires, needs, fantasies, concerns, or insecurities.
It's possible that you and your wife may be ready to try swinging. But it sounds to me like you are not yet in a place where swinging is a good idea. Swinging can be a great addition to a solid, loving relationship that is built on a foundation of trust, and open, honest communication. It might be great for couples who are healthy and happy, and who have approaches to sex that are open to consensual non-monogamy. Your wife's current turmoil, it seems to me, is not the best stable starting point for entering into a swinging situation- which, after all, adds some stresses to a relationship all on its own. BTW, are we sure her situation is a midlife crisis? Maybe we reserve judgement on that diagnosis, and let a professional decide that.
Good luck, and keep talking!