Sundahl, a helicopter pilot, was transporting officials in Kabul when Taliban forces struck his aircraft and killed him. He was 44.
Spears and Sundahl reportedly dated briefly in 2007 after meeting in Alcoholic Anonymous during a troubled period in the singer's life. Sundahl's brother Karl told British newspaper The Mirror via Cover Media, Spears was "upset" by her ex's death.
"John was still good friends with Britney, he talked to her regularly," Karl told the publication. "They dated for a while, but always kept in touch afterwards."
Another insider told the publication Spears was "devastated" by what happened, adding she "thought he was a lovely man."