He stated that he would rather use a spray can clear rather than use a brush on poly over a stain...........
If this is the case, then he can use the rustoleum clear over the stain, whether its an oil or a dye stain, as long as the stain is completely dry before he sprays on the clear. Most enamel and vinyl based "spray bomb" clears contain ketones, tolulene, and xylene, depending on the formulation makeup......all of which are sometimes found in most industrial clear coatings and will not impact the stain.
However, it is always best to do a test sample before applying to the project when in doubt as you suggested.
Now do I think it will be easy to use a spray bomb rattle can clear rather than using a furniture related rattle can clear? NO. I know most non furniture related spray bomb clears are meant for a final coating over a prepared flat surface (painted) and do not contain sanding agents needed to produce a slick finish over wood.