I would say maybe bring it up in a way which isn't like 'I think your husband is controlling' but maybe just mention how you don't see her if he's off? Like 'oh I haven't seen you or spoke much in a while, have you been busy?' or something - so you're not outright saying. It might be awkward if you say it without evidence otherwise and you might lose the friendship.
Maybe it'd be worth seeing if other people close to her have suspicions (or at least think its weird she doesn't text much/isn't available sometimes when he's home?) if you know they're not going to blab or gossip?
Whenever I know he's off and I ask her what she's done she's very vacant. She's openly admitted to me her family don't like him. It seems to have caused a rift between her and her brother. They were close with kids similar ages but although they do still talk you can tell there is tension.
I also know he isn't family orientated. It's a struggle to get him to the kids birthday parties ect.
I personally don't know any of her family so I can't speak to them.
I realise there isn't alot I can do, I'm just a born worrier ?.