I've applied Shellac over Minx on myself just to test it out and it lasted for weeks... I was so bored with the design in the end and took them off before they wore away after about two weeks.
I applied the Minx as normal and then applied a layer of Base Shellac, ensuring to seal the edges as usual and then a layer of top coat, again sealing the free edge and cure for 2 mins and wipe with isopropyl alcohol.
Of course, you will have to ask your client to return to have the removed as they will need to be soaked off.
I don't really see the need to apply Shellac over Minx, as I usually Minx toes and they last for weeks anyway and most clients are happy to have the Minx on their fingers for 3/10 days and be able to remove them themselves at home.
HTHs :hug: