the issue you will have with stone walls, even if polished smooth is the dry surface. which will mean you need some sort of bonding agent. no big deal and cheap to source, easy to apply. but be careful as many have a sheen or satin finish whereas the concrete wall may have a matt finish and will stand out like a sore thumb if you only bond an area or large section of the wall.
I am sure spandex now sell high tack multi-surface vinyl in solid colours for cutting graphics as well as digital print vinyl in short part-rolls. might be worth a try if just a small job.
otherwise prep the wall and install the graphic and just advise the customer of the possible outcome.
once fitted I would be sure to heat and roller with a firm sponge roller to promote better adhesion. particularly if there is any sort of texture to the wall.