Everyone should keep in mind, her concern was likely due to someone telling her she needs one...possibly someone who sells them.
The forum allows other non-financially benefiting view points...so saying she will feel better because she was scared by a exhaust brake salesman is a bit of a self fulfilling prophesy.
Again, they are nice but most certainly not needed in this situation and the cost relative to the value of the truck is a bit high unless she demonstrates an unusual situation (which so far she hasn't) that would change it from "nice to have" to "necessity".