Here's a link to a demo for a product that is available at many RV supply stores.
I used this kit to repair cracks in the bottom corner of black tanks of 2 different motor homes without removing the tanks. The first one held for the 14 years and 150,000+ miles that I owned the coach. As far as I know neither patch has failed yet.
If you friend's leak is in an accessible location this will be the least expensive repair and it's an easy DIY project.