Without being able to see the remote, there are three possible solutions which spring to mind. Firstly, contact the people from whom they bought the property, or place an inquiry through the sales agent. Ask if they have the manual, or if they can explain how the remote functions.
Another possibility is to take the remote into a local store that installs and repairs air conditioners and ask how the remote works. Presumably when she bought the house your daughter was told that the unit was in working order, so if after a sales or service assistant has shown her how to operate it, the air conditioner still won't function, she need to tell the house agent about it.
Finally, she could use the link below to the Mistral website and send them a query and also ask for a manual. I hope this helps!http://homeappliance.manualsonline.com/ex/product/headline/m/mistral/d/air_conditioner/p/MSS6.5