I would put her in a pull up or no underwear, although I do have issues with no underwear. I would not have her borrow other's clothing, including my own children's. I used to lend out clothing, but then I was not getting it back and the other kids would show up wearing my kids clothing.

Send mom a text, when you can, asking that she add underwear to the bag as it seems to have been missed. And let her know her daughter is in not in underwear or has a pull up on because she had no clean underwear.