“Part of a person’s excellence in Islam is to leave alone whatever does not concern him.”
In the same vein, if someone asks you for something unlawful— even if he implores you by Allah—do not give it to him. Imagine he says, “I ask you by Allah to give me such-and-such so I can buy cigarettes.” Do not comply, because he is seeking your help in committing something forbidden.
The principle, therefore, is: whoever asks you by Allah, give to him—provided the request is not for something unlawful and will not harm you. If granting it will harm you, then do not give, for the Prophet (peace and blessings be upon him) said, “There is to be neither harm nor reciprocating harm.”
He also said, “Whoever shows you kindness, repay him.” Meaning: if someone does you a favor—by assisting you, letting you use something of his, or any other act—then repay him. Give him whatever you think matches his favor. If you have nothing with which to repay him, or if he is someone it is difficult to repay—such as a king, a minister, or another dignitary—then pray for him until you feel you have adequately repaid him.
“And whoever invites you, accept his invitation.” If someone invites you to his home for a banquet, whether lavish or simple, accept—so long as attending does not harm you. If going would harm you, do not attend. Likewise, if the host is someone you are religiously obliged to boycott, decline his invitation. Or if his wealth is unlawful and you see that refusing might prompt him to abandon the unlawful, then decline.
With respect to the wedding feast (walīmah), however, the Prophet (peace and blessings be upon him) said, “Whoever is invited to a wedding feast and does not attend has disobeyed Allah and His Messenger.” Accordingly, when the groom invites you to the walīmah, attend—unless doing so will harm you or there is an objectionable practice taking place. If attendance would harm you, you are not obliged to go. If you know of an objectionable practice and can change it, then attend and change it; otherwise, do not attend.
And Allah is the One who grants success.