Wait wait wait. This sounds sketchy. If the grandfather owns the truck, it is his right to determine the course of action.
It sounds like she is trying to keep this from him, which is NOT cool. Why else would you need to sneak up at night? This sounds dumb. Sorry.
Talk to the grandfather and ask him what HE wants done to HIS truck. Ownership is powerful and if she doesn't actually own it, you don't have a right to do a damn thing to it without HIS permission.
I get that you are trying to help a friend, but why not get a REAL lay of the land first?