This is the story with many professionals in the industry secnario these days - especially in Indian Management context.
It is here where inter-personal relationships, good net-working and strong ommunication with lot of assertiveness helps which is essential. It is also here individual's risk taking ability counts and standing upto conviction helps. CRYING BABY GETS MILK it is said and it works. He has to time and again bring up the issue with higher management whenever it is feasible without looking to be "Complaining". Quitely explore the possibility of quitting the job. Objectively and positively assess whether the work-load really means lot of stress or extra hours ?
Now, who is the boss among the HR group ?
If he is, then atleast, he should distribute work among his colleagues and monitor them. Have a strong assessment of performance there. If he feels they are medicore, he should make proper assessment of their work and at least exercise control and show assertiveness there. Management is perhaps is not sanctioning him any additional work-force because it amounts to running away from the responsibility of assessing his own team. If you, and probably you rightly say so, if he needs to learn assertiveness he should start working on it from his department.
Your other questions are situational and organization specific and can not commented in general sense.