i'm not a professional in anything hair and beauty related but had a think about what i would write in your situation.
if staff had realised at closing time firstly i would make it clear to both girls to stop the drama and accusations right away, i would make it clear that they are not to come to work with a bad attitude tomorow as you don't want clients picking up on the bad atmosphere. i would actually go through the till myself to make sure it's not something as silly as miscounting. if there was money missing then i would ask both what their versions of events were. i would to be honest either take notes myself and ask them to read and sign (just their own version obviously) or ask them to write up at home what happened or even both, i would also write my own version of events down. going off what was said about what treatments where performed and what products sold that day i would go through the appointment book as again it could be something like a no show that hadn't been cancelled off. could either have taken money out for lunch if for eg. a member of staff was busy and didn't want to interupt a treatment or receptionist was away from the desk for whatever reason - just asking them the question could jog their memory. as the manager was on holiday i would leave a message saying that money had gone missing from the till and that you had asked both members of staff to write their version of events down and that until she returned you would keep a closer eye on what was happening in the salon - the last thing people want is have hasle as soon as they return but the last thing you want is to forget to mention it to your manager and your manager find out a few days/weeks later or for money to continually go missing, it doesn't look good on you at all. you could suggest you an your manager go over what has happened together in private as one of you might of noticed something but didn't think anything of it until now and could maybe even work together at keeping a watch for what's going on/picking up on anything different, being senior therapist who was left in charge you will have experience and be in a trusted position and will know the other therapists. remembering to remain neautral at all times and to not let it turn into some kind of bitching session. ultimately it's down to the owner to deal with what happend in her salon.
like i said i'm not a professional but they are just some of the ideas that i would do, i'm sure somebody with far more experience will come along who might actually have had something like this happen before to give you some better advice.
x