DEFT Sealer, VARATHANE stain both oil based

I just noticed you said cabinets were cleaned and stained. That isn't how things should be done unless the cabinets never had a sealer or poly coat to start with. A finished cabinet would have stain and poly or varnish. Once the poly of varnish is on the wood you can't just CLEAN and then expect stain to soak in like it should. Altenweg is looking in the right direction, at your painter. He did something incorrectly but its hard to know what since there are so many ways to do things wrong, only a few ways to do it right.

I blame it on the oil-based sealer for the yellowing.
Oil based paint versus latex based paint
All About Oil Based Varnish | Article | Woodworking

If you have Ceiling Fans, and you have white blades, Yellowing from a Smoker is Typical.

Exposure to light causes the yellowing. The inside of the cabinet doesn't get the constant exposure to daylight. The problem is with the product used to seal the cabinets. What kind of wood are the cabinets made of and what color are they stained?
You said the painter said the cabinets are on the cheap side... if they are made from plywood then it is either birch or pine. The painter should have known what he was dealing with before attempting to produce a certain result. I blame the painter and don't let him blame it on nicotine.

I would agree.