I too am very uncomfortable with how people are taking something that doesn't look great on paper and imposing a narrative of abuse on someone who appears to be stable and happy who has never hinted at or claimed abuse occured. Would I be happy with the age gap for my own kids? Of course not. But that's not the same as knowing grooming or abuse took place in this specific relationship.
It is plausible that he had some platonic contact with the family over the years, started to develop feelings for her at 16/17, chose not to act on it and keep a distance (the narrative of Once where he walks away and leaves her be would suggest this - keeping in mind the character was early twenties and separated from her husband), then those feelings reigniting during / after a project when she was an adult and capable of choosing a relationship for herself. It's reasonable doubt, and as someone else stated this isn't court, he's not on trial, the supposed "victim" has never given any indication she thinks or feels he acted inappropriately toward her nor made any claims about this.
Perhaps my discomfort arises from growing up evangelical - these abuse narratives were very much placed on anyone who was LGBTQ, regardless of how much the people themselves insisted that no - no abuse had taken place, they were not "hurting themselves" via ignorance or power structures or willful sinning, that their relationships were not hurting them, no their dad didn't abuse them therefore they turned out gay. The flavour of internet talk about this situation feels very similar.
Yes she was a child when they met. There's no evidence and no allegations from her that he pursued a romantic relationship with her prior to adulthood. To me it's hugely disrespectful to her to keep insisting that he did when none of us can know this. Should she come out talking about him doing so - I would believe her and support her. Until then I will let her be.