The right answer and the easy answer have no relation usually.
If you honestly think that was her canoodling, then would tell my bud, "Hey, it's possible I hallucinated or am wrong, and you know I'd never want to cause trouble in your marriage, but I swear to God unless your wife has a twin I saw her with some guy in the cafe. Isn't she in England? I'm as confused as [bleep]." Then after a few beers, however he wants to address it, stay out of the way but for the being a friend part. Put it respectfully, and hope to be wrong is about all one can do, isn't it?
This type approach is good.
Do NOT accuse anything. If there is some rational explaination for the incident, you won't be the bad guy. If you wrongly accuse, she will never let the relationship be the same again.
