Someone I know told me that Guru Nanak Dev Ji's guru was Bhagat Kabir Ji. Is this correct? Also, is Bhagat Kabir considered God?

No please. The First Guru Person, Guru Nanak Sahib, in his discussion with Sidhs, had made it clear that his Guru was ‘Shabad’, when he said ‘ਸਬਦੁ ਗੁਰੂ ਸੁਰਤਿ ਧੁਨਿ ਚੇਲਾ॥’. Guru Nanak Sahib had collected the Baani of Bhagat Kabir ji, during his Udasi, which was included by the Fifth Guru Person, Guru Arjun Sahib in Pothi Sahib. Since the Baani of Bhagat Kabir ji is, included in Guru Granth Sahib, therefore, the Sikhs give him, the same respect, as they give to Guru Sahiban.