A heavy adjustable dumbbell. Sorry.

For someone with a year or more of consistent training, sure. For the absolute beginner, a 20 kg adjustable dumbbell is a $200 wedge of metal that ends up under the bed because the movement patterns it requires (clean and press, weighted lunge with proper depth) are not where someone starting from zero should be playing.

The thing they actually needed was a $5 resistance band and someone showing them how to do a proper squat. Then six months later, the dumbbell. Order of operations matters.