It is not essential.
But, if you were in their shoes, would you want someone to tell you?
You can't control the fallout from telling them.
That fallout is not your responsibility.
But, the fallout from not being told. I know that all too well. It was awful, long, pernicious, and destructive.
My side of this...
I spent over 9 years struggling with the ghost in my marriage, which was the aftermath of an affair that lasted a few weeks and my FWS's issues being projected on to me. 9 years of thinking that I was the problem, being told in some instances that I was the problem. We even went to MC before D-day, and it did not help. After D-day...with a new MC who knew there had been an affair, a very different story emerged.
Had I been told initially, by some of the people who knew, I might have divorced my wife and run off with a woman half her age , I might have had a revenge affair , I might have killed myself or someone else , I might have bought a nicer car for myself instead of always making sure my wife had the nicer car , but that all falls on me.
Instead, I spent 9+ years dealing with shit I didn't know about, thinking it was something else, and that I was the problem to boot. I almost left my family. I was looking at jobs 100 miles away, thinking I could be close enough to maintain contact but far enough away to not be an issue.
Then, when it was to the point of happening, something else happened 14 years ago, D-day that is, guess what? Those people who didn't tell me, "friends", I don't talk to them, they don't visit my house, I don't even know if they are alive and I don't give a shit about them. I'm still married to my FWS, but we don't have friends that are not "friends of the marriage" any more. After D-day, after the real story came out, after the IC and MC actually had the truth to work with, after my FWS started dealing with her issues instead of blaming me, we cut those people out of our lives completely.
Those who didn't tell me, for a colorful way of putting it, "If they were on fire, I wouldn't even stop to piss on them!!!"
By the way, that applies to the other BS as well.
She didn't tell me, and she knew at the time, and she suffered at the time, but she didn't tell me, so I have no empathy for whatever shit she went through.