What is a ruling regarding a person who quotes Hadeeth to get someone to do what he wants and yet when someone tells the person who quotes the Hadeeth in the same way, he acts as if Islaam doesn't apply to him at all. He argues that we shouldn't be so fanatics and that we should be relaxed or we will end up punishing ourselves. I ask this question on behalf of my friend who is stubborn.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions

A Muslim should fulfill the commandments of Allaah and His Prophet whether he likes it or not. Allaah says (interpretation of the meaning): {The only statement of the [true] believers when they are called to Allaah and His Messenger to judge between them is that they say, ‘We hear and we obey.' And those are the successful.}[24:51].

However, it is sincerity to Allaah and seeking His Pleasure that makes a person fulfill this. People who are disputing among themselves have to adhere to Islamic legislations for judgment when they find they are being swayed by their desires. Not abiding by Islamic legislations in preference to one's desires is the case of the hypocrites. Allaah says (interpretation of the meaning): {And when they are called to [the words of] Allaah and His Messenger to judge between them, at once a party of them turns aside [in refusal]. But if the right is theirs, they come to him in prompt obedience.}[24:48-49]. Therefore, it is an obligation on a Muslim to abide by the truth in all cases and not deviate from it. Accepting the truth is the correct way even if it is called fanaticism, and rejecting the truth is a sin and evil even if it is called tolerance. What Islam legislated is easy in itself; when the conditions are difficult for a Muslim to fulfill an Islamic command, he finds that Islam makes allowances so that the matter becomes easy. For instance, a sick or travelling person during the month of Ramadan is allowed to eat. There are other permissions in religion like this one. However, there are conditions for such permissions of which the people of knowledge are aware. 

Allaah knows best.