Could have happened at the restroom or if she put her purse on the floor next to her chair.
Such a common thing.
Just saw it on the news....it was an outdoor cafe. Apparently the SS saw it, but they don't leave the girls to chase a thief.
More likely she put it down by her chair part way under the table. As the waiter distracted them with his charming way, he signaled to his partner to come in for the steal.