Can you speak to your insurers and see if the onus lies with you to prove your innocence or neighbour to prove your guilt.
Surely they would need to prove guilt? They could accuse him of anything - kicking their dog, stealing their milk etc. Just because you claim something happened doesn't make it a fact unless proven otherwise.
I wouldn't give them the insurer's details initially. In theory, the insurer should laugh them away, but some do take a 'no smoke without fire' approach and increase premiums, even if there was never any fault on the policyholder's part.
I'd ask them for photographic or video proof - if not of the damage actually being done, of the scaffolding being up against their vents (which it wasn't) as anything could have caused the damage - quite often it's just weathering and general wear and tear.
I'd probably make out that I was trying to protect them, as insurers receiving what they have reason to believe are fraudulent claims (i.e. if no proof or circumstantial likelihood can be shown) will often take it to the police. That would be unfortunate, wouldn't it, over a 'misunderstanding' concerning a few pounds' worth of fittings that can be expected to need replacing eventually anyway?
I definitely wouldn't just replace the cheap fittings for a quiet life, as it could well come back to bite you. If he does that, they will treat it as an admission of guilt and word could spread - "You need a scaffolder? Well, whatever you do, don't get Robinson's - they damaged part of our/a neighbour's roof structure and had to be threatened with a claim against their insurer before they put right the damage they caused. If they're careless in that way, would you trust the safety of their scaffolding?"
Plus, as we find so often with CFs, they're never grateful and they mistake kindness and goodwill for weakness and see just how far they can push it. Once he replaces the vents, they'll start looking for any other building-relating improvements on their wish list (whether they're already worn/broken or could be 'with a little help') and blame him for them, demanding that he 'fix' them. After all, he's already admitted carelessness and liability for the vents....