He rightly pointed out that the reason why he has trouble with playing in advanced keys is that other than playing scales, he had very little experience with these keys. So the only practical advise that I could give to students like him is very simple - play as many pieces with lots of accidentals as you can find.
Of course, simply picking the prelude and fugue with 6 sharps or flats from the Well-Tempered Clavier by Bach doesn't help very much. The music is too complex too begin with and you need a nice system in order to play in all the keys.
It's best, if you could find a piece or an exercise already transposed to every key that exists in a logical order with ascending number of accidentals (0 accidentals, 1 sharp, 1 flat, 2 sharps, 2 flats, 3 sharps, 3 flats etc. until you reach the keys with 7 accidentals).
In order to help him and other people who haven't had the experience with playing pieces with lots of keys signatures, today I have transposed a chorale harmonization of Wie schon leuchtet der Morgenstern to every single key of the major mode.
I hope you will enjoy playing these transpositions not only because they will definitely help you in getting to know the keys with lots of accidentals better but also because this chorale is traditionally known as the chorale sung (and played) for Epiphany which is today.
As always, in order to see the best results of your practice, I recommend playing each version of the transposition at least 3 times in a row correctly and fluently before advancing to the next one.
A final thought: if you think that a special course with hymn settings like this one transposed in every key would be helpful to you, please let me know.