Should he give water to a thirsty man when he fears that he himself may become thirsty in the desert?

Ibn Hajar al-Haythami was asked this question and he
replied: 

Two views were mentioned in al-Majmoo’, in (the
chapter called) al-Muqaddam, and I do not know of anyone who
stated that one of them was more correct than the other. The one which
appears to be more correct is that he should give water to the one who
is thirsty at that time, if there is the fear that he will die of thirst,
because his death is certain, unlike the one who has the water, who
has the chance of finding more water later on. But if he is in an arid
land where he despairs of finding water and he thinks it most likely
that death is inevitable if he gives away the water that he has with
him, then the matter is subject to further discussion, and it is more
likely that giving the water in this case is not obligatory. The same
applies if the thirsty person fears that he may lose a limb or become
sick etc. because of his thirst, and the one who has the water fears
that he may die later on, then it is also more likely to be correct
that he does not have to give away the water.