That’s an awfully generous thank you gift! A 2002 Buick Lesabre’s battery location is harder to find than your average vehicle. It’s not under the engine or in the trunk: it’s actually on the passenger’s side of the vehicle, underneath the second row of seats.
To access the battery, you will need to remove the second row of seats. Go to the passenger side of the vehicle, and pull up on the lip of the seats. Go around and do the same thing to the driver’s side, and the bottom part (the part you sit on) of the seats should pop right out.