iPhone suddenly dim on upper left corner

Usually this is caused when the screen has been replaced (probably your friend had it done before) and the screen cables are not fully connected.

There are other rare causes where it is caused by the screen having a faulty backlight instead which would require getting the screen replaced to fix the problem.

Replugging the screen cables most of the time will fix this problem. If you are planning to reseat the screen connectors you must make sure to disconnect the battery first because the backlight circuit is very easy to blow up on the 6s models.

Follow up to step 19 of this guide which is where you unplug the screen display ribbon cable from the screen, the other cables do not need to be unplugged:

iPhone 6s Plus スクリーンの交換

Otherwise it’s a bigger issue with the screen connector on the board or the backlight circuit which either require taking the phone to a reputable shop that does board-level repairs / microsoldering to get it fixed.

TL-DR: First take it to a reputable phone repair shop to get them to try fix the problem via reseating screen cables first, then a screen replacement, last resort board level repair.