My advice would be to start with simple pieces that are just a little too difficult for you to sightread with fluidity. Since you have a good musical memory, always try to read a few measures (1 or 2 if you can) ahead of where your fingers are. Good luck!Also, quantity makes a difference with tasks like this. The more you sight read, the better you'll become.
And how must one practice, in your opinion? I learn so slow... hahaha i have good memory, but i reading is...
One must practice with intent and focus on the specific element that is causing one difficulty.
Sightreading is a lot about pattern recognition.
It doesn't really matter WHAT you practice to improve your sight reading.
I also can't read chords. [...]I'm not even sure if it's possible for people to read a chord while sight-playing. Do people have to stop at each chord (4+ notes) in a piece to work out fingering or can one learn to look at a chord and one's fingers immediately land on the right notes? How is this difficulty overcome?
And pattern retrieval. It is far more efficient to retrieve a thoroughly learned pattern from memory than do prima facie sight reading.
The only true problem in sight reading (as I see it) is the feel of your fingers for the topography of the keyboard ("Where are my fingers in relation to the black-and-white structure of the keyboard?").
I disagree with that. A huge amount of good sight reading is retrieval of well learned memorized patterns. These are fairly specific to various styles. You can be a very good sightreader within a particular style and quickly crash and burn on something new.
No...at least not for me...it's the reading part that gives me trouble. As soon as I know WHAT to play, I have little trouble making my fingers play it without looking.
That means, I suppose, that he's going to need a library full of beginner to intermediate level material.
In that case, you had better practise reading on the sofa with a good drink. Reading is nothing more than translating certain familiar SHAPES of note heads on the paper immediately into a familiar picture of black and white; it's certainly not naming all the notes separately. Ideal is if your technique is so developed, that a certain picture of black and white in your mind also triggers the right fingering.