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.It's probably nothing but my friend comes round to see me weekly with her kids. If her husband is off work though we won't see her at all and she will hardly text when he's at home. We didn't see her for a month recently because he had the month of work.
Her husband is just odd too, he flits from job to job. He spends most evenings down the pub and it's the only thing he can think to take there sons too.
There is other little bits too but I've got a banging headache and can't think ?
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?