1- Mecca existed before Christianity. Yes it was a small village, so one should not expect a lot of archaeological evidence for it and its buildings. But here is a study that proves some archaeological evidence:
2- There are old fragments of the Qur’an dating back to the era of Prophet Mohammad (s), and this proves the Qur’an came from his era. Here’s an article on some of those old fragments verified by carbon dating:
3- Polygamy existed in previous monotheistic religions. What does your Christian friend say about that? Great prophets of God like Abraham and Solomon practiced polygamy.
4- Prophet Mohammad (s) married several women due to noble reasons such as breaking the barriers of racism, bringing interfaith tolerance, teaching others to take care of widows (in the absence of a well established welfare state), neutralizing enemy tribes to bring peace, and encouraging others to free slaves.
5- Prophet Jesus (a) did not have a state or city to rule for him defend it by going to war with his enemies. He would travel from one village to another. He was not settled in one place. Had he had a state/community, he would have defended it. Prophet Mohammad (s) in Mecca lived like Jesus (a). His enemies persecuted him but he forgave them. When he migrated to Medina, he had a community to defend. Also, Jesus was accused by Romans of sedition, so one can conclude that he encouraged his followers to rise against the just state.
Also share with your friend Matthew 10:34-35:
Do not suppose that I have come to bring peace to the earth. I did not come to bring peace, but a sword.
For I have come to turn “’a man against his father, a daughter against her mother, a daughter-in-law against her mother-in-law…
Tell your friend the Day God granted Prophet Mohammad (s) victory in Mecca he freed all his enemies. After 20 years of persecution he forgave them. He did not kill them.
6- Yes there are a lot of false hadiths in books like Bukhari that we reject. Aisha was not 9 when the Prophet (s) married her. This is a historical fabrication.