Nobody knows what goes on between a couple except them. Even if you live with them, are you in bed when one of them makes a snarky remark to coerce the other into sex? Are you aware of all their finances or the way one of them over spends and then makes the other feel guilty for spending on themselves? Do you have any concept of how people behave differently in front of others to when they are alone?
You honestly cannot say you are 100% sure your friend isn't abusive. Nobody knows - sometimes not even the abuser themselves.
My XH seemed genuinely surprised when I told him how hurt I was by the way he treated me for 14 years, he thought he was just being 'rational and sensible' as he had perfectly good reasons why he was allowed to spend money and I wasn't. Would I have told other people about it? No way, I was too embarrassed at being treated like a naughty child and the most hurtful 'jokes' he made about my appearance would never have been shared in front of other people.
If it helps his DW to get the divorce moving by making it about unreasonable behaviour rather than having to wait 2 years to divorce due to separation then I can see why she might want to exaggerate some stuff, but fwiw it doesn't usually make any difference in how things are split between them. If there's no official record of abuse, it won't help her to get legal aid or make any difference to access arrangements for any DCs.
Obviously he doesn't have to agree with whatever she puts on her divorce petition, and he can stall it for as long as he likes but when it gets to 2 years she doesn't need any evidence of being mistreated.
The fact that it has come completely out of the blue is odd to me, if he honestly has no idea that she was so unhappy I'd have to say they weren't that close at all. If she was really acting as if she was still totally in love with him she's obviously some sort of psychopath and he's well rid.