A Muslim should always be kind to his or her fellow Muslims. No one is impervious to sin, but it is the duty of Muslims to help one another stick to the right path and follow the guiding light of Islam.
Answering this question, Dr. Salah Al-Sawi stated,
There is nothing wrong for a Muslim with buying goods from anywhere he likes. He may go to the one who is in his best interests. A Muslim merchant who is committing an unquestionable sin may even be more worthy of his brother commercial transaction. Shirk (associating gods with Allah) is the most enormous of all sins.
If he buys from the store of this Muslim merchant and continues to admonish him or her, he or she might have a change of heart and listen to the advice. Remember that people’s hearts are between the fingers of Allah, Most Merciful, and that Allah will set right the Mahdi (the Muslim leader who will appear at the end of time) in a single night!
So, he should not give up calling this merchant to the right path; he should continue to remind him or her of the teachings of Islam, because he will be rewarded for that.