I don't get the big deal with this. I wouldn't bring this up to the wife at all. If she ever said anything, I would play it off that you took all the texts as a joke, nothing more. I don't know why people would make a bigger deal than this has to be. Just because you are married means you can't receive a text from someone from the opposite sex?
It's not "just a text" its a text with a picture of another man's junk. Their best friends so if it's a joke, and it likely is, this will probably come up at some point so why not just laugh it off to the wife. Get ahead of the situation. If he says nothing and then the wife finds out I'm thinking she'll wonder "why didn't he say anything about this...what is he hiding?". If she's the least bit insecure, and being a female that's pretty likely, that will plant a seed with her that maybe Baloney Sandwich is interested in the friend. Just easier to avoid the drama and say something now.
It is easier to not saying anything at all. If nothing is brought up about it any further, then it is case closed. If you bring something up to the wife now, who says she doesn't overreact like you are assuming she will if she finds out about it later?
If this ("If he says nothing and then the wife finds out I'm thinking she'll wonder "why didn't he say anything about this...what is he hiding?". ") was ever brought up by the wife, that is why I said you would play if off as a total joke, and assume the friend was joking around, as was Baloney.