Papa, I see it done all the time. Doesn't make it right. The problem you will have is the shingles continue to deteriorate and will cup and lift the metal compromising the seams of the lap joints. This will also cause the fasteners to start leaking. You create a bigger problem than you have now. If they install batten strips over the shingles then install the metal, you will have the same issues plus you now have a good air space where condensation will become a problem under the metal panels.
If you want to maximize the life span of the metal roof, strip it to the wood decking, use a high temp underlayment like Tamko Metal And Tile, then put the metal on. Several MFg's make high temp underlayments. Don't use underlayments like Weather Watch, they can't take the heat generated by the metal. You can't take shortcuts when it comes to getting quality roofing systems.