It's a horrible situation. She must be " let go" straight away, no doubt. And friends who took her on at their recommendation must be warned but told that the cleaner is purely a suspect, then they won't be slandering her. It depends after that whether or not they are totally certain and how they feel about telling police, they have to live with their decision. If she works for an agency , it must be told, but they could say they are not happy about having her in the house when they re not there and leave it at that. And no money in house unless in a safe with a code. Ever.
A camera is good if they can cope with the consequences. And if they get proof they can decide what to do. I use an agency, and any small problems are quickly dealt with but I take photos as back up. Not for missing money, but for other things, not major. The agency always wants to know any problems straight away.