"I challenge all of you to bring me the worst deeds you think the Prophet Muhammad did and show me your reasoning. I doubt you'll have success."
I'm not sure what she means by the "doubt you'll have success" part. If she means we won't be able to show that Muhammad committed some of the most horrendous acts imaginable, she's clearly wrong. However, if she means that we won't be able to convince her that Muhammad's deeds are horrendous, she's probably right. Muslims take Muhammad as the pillar of morality. Whatever Muhammad does is, by definition, good. Hence, it's nearly impossible to convince a Muslim that Muhammad's actions are evil, even when his actions include rape, torture, murder, polygamy, slave-trading, robbery, attempted suicide, racism, prostitution, and child molestation.
Of course, we have a ton of examples to choose from. I'll start a list, and others can add to it. Here are some of the things I find particularly egregious (in no particular order).
(1) Muhammad had sex with a (prepubescent) nine-year-old girl.
(2) Muhammad married more women than his own revelations allowed (he received a "special" revelation giving him the privilege of violating Surah 4:3).
(3) Muhammad married the wife of his own adopted son.
(4) Muhammad had sex with his slave girls.
(5) Muhammad attempted suicide.
(6) Muhammad delivered a revelation to his followers promoting polytheism.
(7) Muhammad supported idolatrous pagan practices such as kissing a black rock and bowing down to the Ka’ba.
(8) Muhammad ordered his followers to assassinate people who criticized him or his religion.
(9) Muhammad beheaded hundreds of Jews for trying to defend themselves once they realized he was eliminating them.
(10) Muhammad started a war with Mecca when he had a chance to live in peace in Medina.
(11) Muhammad committed numerous acts of terrorism.
(12) Muhammad enslaved thousands of people.
(13) Muhammad allowed his followers to rape their female captives (though he took the most beautiful captive back to his own tent).
(14) Muhammad told his followers it’s okay to beat their wives into submission.
(15) Muhammad hit his wife Aisha.
(16) Muhammad broke an oath to his wives (after receiving a "revelation" giving him permission to break his oaths).
(17) Muhammad wanted to divorce one of his wives once she had become old and unattractive.
(18) Muhammad showed favoritism among his wives.
(19) Muhammad told his followers that women are stupid and that their testimony is unreliable.
(20) Muhammad tortured people for money.
(21) Muhammad supported his religion by robbing people.
(22) Muhammad preached a message of violence, oppression, and cruelty.
(23) Muhammad taught his followers to believe in a god who loves only them, and no one else.
(24) Muhammad ordered his followers not to be friends with non-Muslims.
(25) Muhammad commanded his followers to fight and subjugate non-Muslims.
(26) Muhammad said that Ethiopians have "heads like raisins."
(27) Muhammad allowed his followers to have sex with prostitutes (though, according to some sources, he eventually changed his mind).
It's a start. As always, I'm happy to help Kim learn about her prophet!