Single speed and fully rigid is the way to go at this price point, with a rigid steel fork. If you do your research you could put one together fairly cheap. Budget for decent tires though.
Dear Lord, that’s a sure-fire way to make someone hate mountain biking if THAT is all the advice you’re going to give. You also need to at least specify that it better be a 29" bike and the person should be willing and able to play with different gear sets as well as be able to properly dial in tire pressures (and they probably should go tubeless). Without those things, well, it’s just too easy to end up with a bike you absolutely hate for so many reasons it isn’t even funny.
–Donnie