according to the rule you mentioned, "he still have her/hair" would be :
Hij heeft haar nog (her)
Hij heeft nog haar (hair)
these feel quite reasonable. However when I tried to apply the rules to some other examples, it starts to be a little strange, could you help me with it?
-1. hij drinkt het langzaam.
-2. hij drinkt langzaam water.
is the second sentence a correct one? It feels a bit strange.
' Wees blij dat je nog JOUW haar hebt ' makes sense but then again it's extra/unnecessary info because the sentence already talks about the same person (je) and 99,9% of the times you use that sentence, you refer to the person being bald or not (having their own hair or not). 0,1% you refer to the person having some other person's hair (be it their head or on their hands etc), which would require extra wording to describe more precisely of what's happening