It’s not about “plausible deniability” it’s asking a question when you’re concerned, without making a serious but potentially erroneous accusation.
I’d be worried if there were indications that my best friend’s wife was putting him at risk. I care about my buddy, and part of being his best freind is watching his back. If it turned out that his wife was cheating on him for months or years and I didn’t say a word, that would be betrayal on my part. A question would put my mind at ease. And I would hope/assume there was a logical explanation.
When I was a kid, my dad took a family friend to a function. A few weeks later, a friend of my mom’s asked how my dad knew “Sally”… his “date”. There’s three reasons why my mom’s friend would ask a benign question: 1) She was concerned for my mom. 2) She wanted reassurance. 3) She was warning my mom that the rumor mill was churning.
My parents aren’t stupid. You can totally assume that their friend was wondering about the gossip just as much as the next person, but at least she asked a question that assumed there was a logical explanation for seeing my dad out with another woman. My parents weren’t pissed, no one thought it was shit-stirring.
If I know for 100% damn certainty that my buddy’s wife is cheating, I’m going to tell him. If I’m just seeing stuff that I think is really weird/suspicious, then I think it’s better to ask a question about it. If everything is on the up-and-up, then no worries, I’ll be reassured without insulting his wife by jumping to conclusions. If it’s not on the up-and-up, then he can look into it privately, still without me insulting his marriage.
ETA: If I wanted to let him know and totally be “cunning” about it, as you say, I could mail an anonymous letter.