Furniture, Wood and Cabinetry Finishing - Discoloration on stained staircase

My friend (seriously, it's not me!) stained a new staircase with two coats of an oil-based stain. Then he put on a coat of polyurethane, but he can't remember if it was oil or water-based and the can is gone now.

His complaint now is that the surface has developed white and yellow spotting all over, and where you step is very discolored.

My thinking is that he might have used a water polyurethane on top of the oil stain, but I'm not 100% sure.

He's looking to find out how to fix it. I told him he may not like the answer, but I would check for him anyway, especially since I'm not 100% sure of the cause...

Thanks!

It's rarely a good idea to apply more than 1 coat of stain. Waterbased poly/varnish can be applied over an oil base stain but the stain must be good and dry! Woodwork almost always requires 3 coats of poly/varnish to get a decent job. Did he only apply 1 coat?

He might try wiping a section down with a damp thinner rag but I expect the fix will require sanding. Too soon to tell if the stain will need to be redone.

I agree with Mark - one coat of stain and three coats of poly is what I'd do.