I helped my friend in doing a project and she gave me an expensive gift to show me her gratitude. Is it ok if I accept her gift? I knew that the Prophet (Sallallahu Alaihi wa Sallam) said once to the man who collects Zakah that he should return all the gifts which they gave it to him. So what should I do now?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Gifts are means of building good relations between people. This is why the Prophet (Sallallahu Alaihi wa Sallam) recommended it. Imam Malik reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Exchange gifts among you and you will be pleased with one another" . al-Bukhari reported that the Prophet (Sallallahu Alaihi wa Sallam) also said addressing Muslim women: "O Muslim women! None of you should look down upon the gift sent by her she-neighbour even if it were the trotters of the sheep (fleshless part of legs)" .
al-Bukhari also reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "I shall accept the invitation even if I were invited to a meal of a sheep's trotter, and I shall accept the gift even if it were an arm or a trotter of a sheep" .
We remind you here that the scholars forbid giving gifts to judges and people who are like them, to close the door to bribery. The Prophet (Sallallahu Alaihi wa Sallam) said: "Allah Curse the person who gives bribes and the one who accepts them" [ Ahmed and al-Tirmizi ].
To conclude, know that there is no harm for you to take your friend's gift as long as this gift is not a payoff for work that she should have done.
Allah knows best.