This makes me a bit uncomfortable: "She has been paying the truck and all related expenses using her LLC. Can she deduct all the truck expenses under her schedule C?"

in concert with: "She is paying back the owner in a monthly basis."

Because it isn't clear if there is a Lease or Loan agreement between these two parties. At first, it seems like she was making the Payments to the lender, but with both parts, she is paying the other person directly.

Are you trying to write off the Truck Payments?