B.B.
First of all there is NO way to potty train a child in one day no matter how smart or whether they speak in sentences. My daughter has always talked more then other kids in her class, walked earlier, talks in complete sentences and we are going into our 3 rd mth of potty training. Potty training takes time and patience. If you only have her for a week then it will depend on whether her mom is ready to potty train her . Some parents wait longer then others. I originally started to potty train by daughter when she was 18mths old and she would sit and do nothing then we took some time off (until she turned 2yrs old) and now she goes about 50% of the time. She wasn;t ready at that time even though she showed the signs of being ready. Once she was ready she started going on her own. All you can do is get her accustomed to the potty. Take her periodically, sing a potty song and reward her for trying and then reward her for going once she starts. It is a long process and can't been rushed. I would trying using pull ups on her and putting her on the potty every hour. Sing a song and find a way to entertain her so she sits longer and is able to go. My daughter and I have a song I made up and we clapp it encourages her to sit until she actually goes, before she use to sit briefly and would get up and pee within a few minutes. Make it fun. Also, buy a book or movie about potting to help her. Remember you can't do it in a week or a day so her parents need to be on board. I would spend your week enjoying this time with your granddaughter rather then putting her on the potty every few minutes.