A good gift for a person that you barely know?

I'm a girl who is dating another girl for a few months. Today is her mother b-day and her family is planing to do something simple in her house, like cutting a cake and sing happy b-day to her with some friends and family, nothing fancy. My girl asked me this morning if I would like to go tonight to her house to sing happy b-day to her mom. I said "Yes", of course, but I don't know what to buy for her. I meet her mom like a month and a half ago but I had only see her like 2 more times after that. One time we had dinner in a restaurant together, me, my girl, her sister and her mom, but we didn't talk much. I think she likes me but because I'm a girl who is dating her daughter is a little awkward for her. Since I was invited, I don't want to go empty handed but I don't know what will be appropriate to bring her and I don't have much time and money.