The scenario you seem to be talking about here, MrsWobble, is where one partner has been emotionally abusive to the extent that the other has sought solace in an affair. The abuser may then be laying a guilt trip on the adulterer, either to make them stay or to give them the moral high ground when separating. Or, of course, something we have seen here often enough, the adulterer is reinventing a previously happy relationship to blame the affair on their partner's alleged abuse; whilst if Partner 1 really is an emotional abuser they will never admit it to themselves, Partner 2 or anyone else. Unless we all know both parties intimately, it is impossible to judge fairly. Controversially perhaps, I'm going to say that finding the culprit is not really that important.
It's easy to get bogged down in playground-style finger pointing, but blame doesn't achieve anything in the long run. These are two people who are not happy together. They almost certainly never will agree on who is to blame. What matters is that they stop making each other unhappy, ie get out of there and worry about who was worse as an academic exercise afterwards. The law will not assign assets on the basis of who was naughtier (except in very extreme cases), but on what they both have balanced with what they both need. In order to stop their precious assets being frittered away in legal wrangling, or one party trampling all over the other, they need to let go of all such reactions as guilt and righteous anger and look at it all practically.
It probably does matter very much to one party whether they can be forgiven by friends/family or can even forgive themselves. I can only say that time, and their experience with other partners, will show who is right.
On the other hand, if we are talking about a couple who do not plan to separate, it would call for one hell of a lot of work on both sides, starting with the decision to stop blaming and find out what each of them can do to make things better between them. Anything else is a recipe for many long years of misery.