Malunions and nonunions have no much relation to the kengthening amount.
The same happens to surgery risks (anaesthesia, infections).
The only difference is the rom and the stretching on the soft tissues which is significant of course but 5 vs 6cm is not a big difference. Up to 6.5 cm on tibias are considerably safe, more than that is risky.
Personally I'd go for less than 6 cm on tibias as when I did LL till 6.5 everything was almost excellent, only after that things get worse.
he should do external tibias leg lengthening if he wants to do less than 5 cm.
-malunion can be fixed with external ll easily
-nonunion is very unlikely and rare especially for a very small lengthening and I believe he is a young healthy dude.
-only infections are pin site infections with external ll which are easily taken care of and not serious (unlike an infection that can occur in the bone canal with internal rod)
-risk of anesthesia is very low. You are more likely to die in a car accident, spider bite, etc. You can opt for epidural/spinal anesthesia if you want which has been documented to have even less risks than general anesthesia
#1 thing is you should choose a very good external ll surgeon that can fix misalignments and has a good track record.
Here is why external tibias ll is safer than internal femurs. Remember you can also the precise can malfunction or break along with these possible complications:
(just click on where it says "quote" to view why external ll is safer)