Just before Ben meets Leslie's mother Leslie spills coffee on his shirt and it leaves a large wet stain. Seconds later when Ben goes in and meets Leslie's mother the stain is perfectly dry and size/shape of the stain has changed; then as Ben and Leslie's mother talk the stain changes shape and size again several times as the scene progresses.
It's inconceivable that after nearly a year or so in Pawnee, and spending considerable time with Leslie, that Ben would not have heard that her mother also works at City Hall and in fact is more prominent than Leslie. Leslie's mom is the only family she has and they are not estranged. Additionally, Leslie has a photo of her mom behind her desk.
No one suggests swapping Tom's and Jerry's new assignments even though each has an assignment that is far better suited for the other.