Should he pray behind one who did not wash his hands again when he washed his arms in udu?

Firstly: 

In the answer to question no. 103694 we stated that what is required is to wash the arms from the tips of the fingers to the elbows, and that if they ware washed from the wrists then wudu is not valid. This is the view of the majority of scholars, except the Hanafis who say that the wudu of one who does that is still valid. 

Secondly: 

It is valid to pray behind a Hanafi who only washed the hands at first and did not wash them again with the forearms, because it is an issue concerning which the scholars differed, and the ordinary Muslim is excused for following whichever scholar he follows. 

It is valid to pray behind one whose situation you do not know, and you do not know whether he washed his hands in the correct manner or not. It is not appropriate to doubt the worship of a Muslim, rather you should assume that he is doing it correctly.. 

You should explain this issue to your brothers with wisdom and beautiful preaching, but if they insist on the view and cling to the Hanafi madhhab or the view of the one who issued a fatwa to them, then do not argue with them, and do not feel any reservations about praying behind them. The imams continued to pray behind one another even though they differed concerning minor issues. Hence they regarded it as valid to pray behind one who thought that wudu was invalidated by eating camel meat or touching the penis and other matters concerning which the scholars differed. 

Ibn Qudaamah said in al-Mughni (2/11): As for those who differ concerning minor issues such as the companions of Abu Haneefah, Maalik and al-Shaafa’i, praying behind them is valid and is not makrooh. This was stated by Ahmad. That is because the Sahaabah and Taabi’een and those who came after them continued to lead one another in prayer, even though they differed concerning minor issues, so their action is equivalent to scholarly consensus. And because the one who held a different view is either correct in his ijtihaad, so he will have two rewards, one for his ijtihaad and one for getting it right, or he is mistaken, so he will be rewarded for his ijtihaad and there is no sin on him for his mistake, because it is forgiven (as stated in the hadeeth). If he knows that he has omitted an obligatory part or condition that the person praying behind him believes in but the imam does not, then according to the apparent meaning of Ahmad’s words it is valid for him to lead him in prayer. Al-Athram said: I heard Abu ‘Abd-Allaah when he was asked about a man who led some people in prayer, and he was wearing a fox-skin. He said: If he is wearing it on the basis of his understanding of the hadeeth, “Any skin that had been tanned is purified,” then he may pray behind him. It was said to him: Do you think that it is permissible? He said: No, we do not think that it is permissible, but if he thinks that then there is nothing wrong with praying behind him. End quote. 

And Allaah knows best.