I think what exactly he did/didn't do is deliberately vague; he clearly has some guilt about the incident, and it could be that he merely feels guilty about not trying harder to help, but there's a definite unease that there's more to it than that.
That thread of women continuing in relationships with men who have hurt them or do hurt them runs through the show, doesn't it? Even if Johnno didn't hurt her, actively, the indication is that he was/is capable of doing so, even if the relationship is/becomes healthy. Think of Robin's mother's relationship - it's not dwelt upon, but her boyfriend (or was he her husband?) had punched a hole in the wall. He loves Robin's mother - and Robin - but he's still capable of that violence, and I couldn't help thinking about it, even when Robin and he were comforting one another in the wake of Robin's mother's death and when my sympathies were fully with him.
Campion's interested in those complex, muddly relationships, I think, and there's a similar complexity in In the Cut, where Ruffalo's character is blatantly and horribly misogynist on the one hand but also exactly what Ryan's character wants on the other; he's both misogynist and tender.
I love that position Campion's puts us in as viewers - a similar position to the women themselves, I think, where desires are complex and not completely logical; we see the threat, but we also want any two in a given relationship to be together, Robin and Johnno, Ruffalo and Ryan, Robin's mother and her partner.
I've been thinking about GJ's camp in regards to this, too; the women make no real effort to keep men out. They could, if they wanted, to, post guards and warn men off with guns or something, but they don't. They don't seem particularly wary, in spite of what they have all been through in relationships with men in their past. So the camp is a such a weird mix of women being women and living communally and yet not really being exclusive; men can come if they like. The camp is not "safe" from men, and it isn't necessarily meant to be - because the women have no interest in shoving men out of their lives - men are a part of who the women are and what they desire, however complicated and perhaps dangerous that relationship might be.