How do I get a dog to trust me after they met me initially?

I had a situation today that I wondered if you could give me advice on. I went to meet a dog I had already done a meet and greet with. She was fun-loving and energetic. I went to walk her today for the first time and she seemed extremely nervous and scared without her owner there. I understand where she is coming from due to a stranger, coming into her home and talking to her. She wouldn't let me get near her and was barking and growling the whole time. The entrance is through a garage and upstairs to the main part of the house. I tried to talk in smooth tones and to move slowly up the stairs and into the house, but she kept barking and growling. She was standing guard at the top of the stairs. I called the owner and asked if there were any treats or toys I could use to gain her trust, but everything was upstairs in the house. I even put the owner on speaker phone to help her dog calm down. We are going to try again Thursday with Bella's owner there. Is there anything I could've done differently or that I can do Thursday to help her trust me? I am suppose to pet sit her later in the year. Thank you for your time!