OP, I do understand your concern at reporting and the possible/likely reprisals of your client becoming more isolated and even lying to professionals if they did intervene.
However, the alternative is serious injury or even death, either due to physical injury or unmet health needs if the perpetrator prevents her from attending appointments and having the care she needs. Not to mention the lifelong impact on the child living in this environment.
Please log with social care in your area, both children’s and adults. You can do so anonymously. Make sure to include that she is about to have an operation, and you don’t have eyes on her now so she is more at risk. Also include that he checks her phone.
There are lots of ways professionals can discretely check in with her and give her the opportunity to talk while making it seem like procedure - GP asking how everything is at home, hospital asking if she has had any recent injuries or illnesses before the op (and noting any bruises or responses).
I think I missed the age of the child, but health visitors/nursery/school can all keep an eye out.
They may all already have concerns but no evidence. Neighbours may have reported concerns. Your referral could be the last piece of the puzzle, or the first - but it will help keep them safe.
Also consider the risk to you in visiting. I’m not telling you to stop, but make sure someone knows the address and when you will be there. Familiarise yourself with silent solutions or download an app such as Hollie Guard.
You are lone working in an abusive household.
Agencies will do all they can to keep referrals confidential and won’t want to risk her losing vital support.