So here's an update. I spoke to my friend again, and he had 2 regular suitcases which he was checking. In addition, he was holding a regular backpack that could (and did) fit under the seat in front of him.
The bag in question was an overnight bag, which he assumed weighed around 8 kg. It had a small amount of clothing in it, and that was it. When he check-in, they saw him holding 2 more bags, and asked if he planned to carry both on. He said yes, and he would put one in the overhead bins and the other under the seat in front of him. They told him that was against LY policy, and made him check the overnight bag for $200.