Problems unhitching van from Towball

Try loosening the locknut and backing off the adjusting screw, he probably has it set too tight.

Sue. With the very light towball weight on those vans they can be prone to rattling and eventually wearing. I would guess your Dad has adjusted it up to eliminate the rattle, and as others have said, it is now too tight to allow easy removal. We,as Dads, don't "interfere" we just have our precious daughter's best interest at heart and sometimes go overboard a bit. lol. With the release lever up, undo the adjuster screw locknut. Undo the adjuster screw a bit at a time until the coupling comes off the ball and lock it in this position. Keep a bit of grease on the towball to lubricate it and all should be good.
Maybe give Dad a hug as well then for caring, xxx. Bob