Bismillahir Rahmanir Raheem,
Wa Salaatu Wa Salam Ala Habibuna Wa Nabiyuna Muhammad.
First in discussing with him I would advice you to purify your intention as I advise myself, insha'Allah.
I would ask you to remind him that we are all humans with a common goal, i.e. to worship God, so when we discuss this matters, we should start from a point of wanting to gain knowledge and clarify matters and not score points.
Next step I would exhort you to record and take things step by step with the individual, for example, with this statement that you have poster dear brother, analyse it and put forth your observation, these are mine;
First here is his statement:
I do not know what God will or will not accept. The only one who does know what God will or will not accept is God himself. And we will find out what that is after we die.
Nothing for certain because what we know of the Prophet (pbuh) comes from fallible and imperfect human history filtered through the mutterings of more than a century of fallible and imperfect human scholars. The Quran on the other hand is of divine source (I believe).
Complexity is not an indication of accuracy or rigour. What are these comprehensive steps? Who takes them? As determined by who?
Considering that the family and companions of the Prophet Muhammad started disagreeing with each other right after the Prophet died I am not very impressed by that. Some give more credence to the indirectly transmitted traditions of Ali, some give more credence to the indirectly transmitted traditions of Aisha. I'd rather give no credence to any of it as a basis of religion.
That's what he said.
Really difficult.
First Observation;
The Quran on the other hand is of divine source (I believe).
The Sunnah or path of the Prophet is of divine source too, does he not know that the Qur'an, which he takes as divine, states, translation of the meaning taken from Introduction to the Sciences of the Qur'an;
<<And We have sent down to you (O Muhammad) the Remembrance, so that you may clearly explain to mankind what has been revealed to them, and so that they may give thought.>>16:44
And who is the source Muhammad will use to explain the Qur'an;
Then it is for Us (Allah) to make it clear (to you)75:19
Muhammad was not a creator of the religion,he did not come up with explanations of himself. Allah makes it clear to Him, and thus the Sunnah, the way of Muhammad is of a divine source. Just like the Qur'an.
Observation Two
The Quran on the other hand is of divine source (I believe).
The second point is from this statement also, it is a question, it seems you assume that due to the divine origin of the Qur'an it is somehow preserved, just because something is originally from God does not mean it is guaranteed preservation. If you believe it does then please show us why. If you believe the Qur'an was preserved please show us the method through which God, throughout history has preserved the Qur'an.
It is my understanding that when one studies history he finds the Qur'an was preserved, by God, with the same means as the Sunnah. Individuals who lived, breathed, studied and dreamt about Islam, individuals who fought and sacrafised for Islam, preserved both the Qur'an and the Sunnah. They were, as it is clear from history, the tools that God utilised in the preservation of these sources.
Third Observation
This observation stemms from this statement:
Nothing for certain because what we know of the Prophet (pbuh) comes from fallible and imperfect human history filtered through the mutterings of more than a century of fallible and imperfect human scholars.
Whilst I do agree that fallible humans cannot produce something perfect like the Qur'an for example, this is not what you are stating nor the point being discussed. What the
real point is, can fallible humans preserve a text which is infallible? I don't see why they cannot.
All these humans done was take what was given by Muhammad, and preserve it. Like for example, if I took a letter from someone, and kept it in a safe, and passed the keys down in generation to my children, after a thousand years, if you open the safe the letter will have been preserved, so I and my infallible children will have preserved a text 100%.
I will insha'Allah speak on the rest another time if I can.
And Allah knows best.