It it's a typical swinging door you can go to your local hardware store and buy a "rubber door wedge / doorstop" (couple dollars) Usually about 4 inches long by 1 inch high and you wedge it under the door.
Or use a bungie cord. Or your husbands belt. Or an old pair of nylons. Or a shoestring or two tied together. Or a chair. Lot's of things as you can see.
IF, on the otherhand it's a "sliding" door:
You can purchase from your favorite hardware store a metal "extendable / adjustable" closet rod. Make sure that it is from 24 to 36 inches.
Basically all it is is a tube inside a tube that when screwed together are expandable up to 36 inches or retracted to 24 inches. Simply adjust to the length and place on the bottom track thus preventing the door from closing. Cost is under $10. """"NOTE***** The aformentioned "rubber door stop" will work here just as well.
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