I think your friend isn't as lucky as you think. The eye/brain should automatically make the focus as sharp as possible so all that you can do consciously is make it worse.

Then again, if your vision isn't perfect there are tricks like placing pressure on the eyeball or squinting which can change the focus, but are probably not too healthy in the long term.

Best advice is to see an optometrist instead. Although, I have found that they aim to correct vision for 6 metres in red-green light. If you're reasonably young and myopic, ask for a slightly stronger minus power to get better blue vision at infinity (at the risk of more eye strain close up). The extra -0.25 makes a huge difference for me, really allows depth perception to "click" properly.

20/20 (or 6/6 in metres) is awful. I didn't know that until optometrists started moving from cards at a distance (where I could choose which line I wanted to be able to read, thank you very much) to the retinal projection thingies most of them use now. The difference is, as you noted, about -0.25 for myopia in the -3 to -4 dioptre correction range, and it's huge. I'll assume that it's a better set-up than the old way for astigmatism (which was never a problem for me), but settling for "normal" vision when there's an option... I don't think so.

If you know a bit about optics it's not hard to derive. The difference between correction at 6m vs infinity is 1/6 - 1/inf = 0.166.. dioptres.

On top of this, one thing that surprised me was the difference in correction between red, green and blue light. The refractive index of the eye varies according to light frequency such that blue is focused approximately 2 dioptres more than red. If your myopic eyes are corrected for red at infinity, you won't be able to focus on any blue light further than 0.5m away! Not really a concern in the natural world but very noticeable in a server room full of blue LEDs or on a screen with blue text on black.

The difference in power between red and green is less but still useful enough for some vision tests. If you can focus on red traffic lights but green ones are blurry, this is why - consider a stronger power as long as it doesn't strain your near vision.

I'm slightly farsighted, and while I can see without my glasses, I have to make a conscious effort to bring the world into focus if I'm not wearing them and headaches eventually result.

Only trick I know is getting the right prescription glasses...