Firstly, I am in Australia so I cannot answer specifically for laws in the UK. However, I am pretty confident in saying that your friend's landlords are definitely breaking the law.
Nationwide, generally you can't point a security camera in a location that a reasonable person would expect to be afforded privacy. In other words, it's okay to record the street directly outside your house, but focusing on a residential window or an enclosed backyard usually isn't allowed.
In some states here in Australia, there are additional laws about recording sound. For example, recording of private conversations without consent is prohibited. This can be a bit tricky to avoid; especially if you live in a tightly packed townhouse or apartment block. If you want to stick to the law, my advice would be to get the landlord to disable the audio and the camera. If they don't I am pretty sure you are well within your rights to lodge a complaint, with your relevant tenancy department.
Bear in mind that the landlord will probably have their own rules and regulations about security cameras in place, but they must be aligned with the law. And it must be in writing when signing the tenancy agreement. However, by law, all tenants have a right to their privacy and generally landlords are NOT allowed to put video cameras anywhere near or in view of a residence. Cameras are only possible in common grounds/body corporate grounds that belong to the body corporate and not one owner of a unit.
If the unit complex is owned by one person, even then the owner has to form a body corporate by inviting tenants of the unit as members, or to hire a body corporate firm that manages all of that. At least the owner/landlord cannot just do what they want without other people knowing about it.
On a final note, the landlord needs to apply common sense when setting up a camera, regardless of what the law says you can and cannot do. For example, you might be technically "allowed" to point your cameras towards a kids' playground, but doing so would be extremely stupid, and it opens them to litigation.
I would advise your friend to seek legal advice from a relevant tenants union and legal-aid center, and if the landlord still persists with their cameras, your friends can offer them a slightly higher bond for example, to cover some damage to their furniture. But even that is stupid because the landlord should have landlord's insurance to cover all of their house and contents.