Is the questioner sinful for abandoning his friend whose circumstances changed for the worse after being subjected to slander, and he started to think ill of others and act in ways that harm them, despite his continued observance of congregational prayers?

There is no sin in limiting oneself to greetings without engaging in extensive conversation with someone whose company is harmful. However, it is advisable to endure it for the sake of Allah the Exalted, and strive to advise them by explaining the danger of ill-suspicion and the ugliness of frowning, and the virtue of receiving Muslims with cheerfulness and a smile.