Good for you for praying this through and following the Lords guidance on how far to pursue this.
In an effort to address her types of statements/questions (because they are common) and to have knowledge around them in order to address them, I’ve listed some of my thoughts below:
I think it’s important to remember where we’ve come from. Ravi has said “that to forget the past is to obliterate the future”.
When considering America’s past, admittedly the conversation around presidents and religion and their outright profession of faith, the ground is a bit shaky. But I think it’s fair to say that many of them came from Christian backgrounds - https://en.m.wikipedia.org/wiki/Religious_affiliations_of_Presidents_of_the_United_States#List_of_Presidents_with_details_on_their_religious_affiliation
There are other sources that substantiate this- https://books.google.com/books?id=VGF3wbzzy9QC&pg=PA322#v=onepage&q&f=false
Lincoln referred to biblical principles, the Bible, God, and the need to do away with slavery as a result of Gods will for that to happen in his second inaugural speech- http://www.abrahamlincolnsclassroom.org/abraham-lincoln-in-depth/lincolns-second-inaugural-speech/
If one had to say where has the decline of morality and ethics come from in the West, it would have to be agreed that it is its departure from the Bible if many of the moral & ethical laws were based on it.
Even a Muslim would agree with this as they hold a high regard for morals and ethics, at least intellectually, like many “Christians”, they can also live pretty thick in the world. So even if they don’t believe in all of the Bible, they would agree with many of its moral and ethical standards, because they believe in many of those themselves. So find ground for commonality with anyone that has questions of the Bible, it’s a good place to move from.
The statement of Joshua and his conquests, here’s an article I thought was a fairly good read: https://www.seedbed.com/7-keys-to-understanding-violence-in-the-book-of-joshua/
Also, Deuteronomy 20:16 needs to be taken in context of all of chapters 20 & 21 of Deuteronomy. The overwhelming support of Gods mercy in these chapters far outweighs positioning Him or Joshua etc. as monsters, it’s the most gracious of warfare terms one can imagine.
The people of Canaan were wicked people who sacrificed babies, and God acted justly against such wickedness, often destroying them and nations like them Himself, with Joshua and the like having very little participation in their destruction, thus the God of the universe was attempting to maintain purely, is He not just to do so, they would say Allah is.
On the issue of human rights, by far the overwhelming amount of countries with issues in humanitarian issues are middle eastern and North African countries:
I think we can see God as a God of mercy many times over in the Old Testament, and this ultimate mercy and grace is shown when God the Son sacrificed himself in the form of Jesus Christ to reconcile man to Himself in such a way that mans evil could be dealt with once and for all, by accepting that Christ died on the Cross for His sin.
I would point someone like her to the person of Jesus Christ as Gods act of ultimate love and mercy and how he died for all people and nations. And how He, time after time, displays gentleness and grace to all.
Muslims battle with concept of Christ as God the Son because Allah is “so great” that they can’t think that a god would subject himself to being incarnated into a man, or even converse with a man (Muhammad received the word of the Qu’ran from the archangel Gabriel). If she’s Sufi she would be an exception, they are more into mysticism etc. and believe that Allah does speak directly to them, but this is not the general Muslim consensus.
So the concept of a god sacrificing himself is seen as weakness, IF seeing power or greatness in the context of the sword.
But when it’s seen through the lens of love, anyone who is almighty and all powerful restraining his power, that is a real display of power and greatness.
And when considering the full extent of all mans sins falling onto one man, that takes great strength and power.
Some of my thoughts to navigate some of these discussions.