They stole tools and other items from the vehicle before pushing it into a river.
When police questioned Lee about her son driving without a licence she kept quiet about the stolen vehicle but police later returned and found the stolen tools.
Lee said she hadn't told police about the stolen vehicle initially because her son had been out of trouble for 18 months and she wanted it to stay that way, Mr Harris said.
Judge Robert Murfitt told Lee that allowing her son's behaviour was teaching him that crime paid.
"If you want to avoid visiting him in prison you need to make sure he knows right from wrong," Judge Murfitt said.
He sentenced her to 80 hours community work and ordered her to pay reparation for half the repair of the stolen truck of $1012.