I lost the whole of my right ring finger somewhere about 12 years ago (I caught my ring on a nail & jumped down some steps - ouch). I work in software & touch-type, so I was a bit concerned about what would happen to the manual dexterity.
As soon as I returned to work & started typing, the other fingers on my right hand took over appropriate keys without any concious effort on my part (giving rise to my theory of distributed intelligence) - and within a matter of weeks I was typing as fast as ever (it only took that long because the whole hand was stiff having been redesigned by taking out the finger way down toward the wrist & pulling the other fingers across to close the gap a bit).
Similarly - my little finger took over all the ring-finger's picking duties on the guitar. A pianist told me that piano-playing would not be a huge problem (except I don't know how). Woodwind might be a pig, but I'm sure an instrument could be cleverly adapted if necessary.

I have never particularly missed the finger, and quite like having a cartoon hand (3 fingers + thumb).
The left hand is a bit trickier instrumentally, but I fully expect that he'll quickly learn to compensate with the rest of the fingers. Just look at Django Reinhardt.
Someone once asked my husband whether he noticed any difference from my having lost a finger. He replied that it made his **** look bigger.
I had the last laugh when he lost a ball to cancer - as I was able to comment that at least it made my hand look bigger. (he's fine now btw)
Tell your friend to keep smiling and all will be fine.

Kris