Hi, Farfalleus - and welcome to the forum.
I think part of the challenge here is that Lewis’ critics are right. His trilemma failed to consider a fourth option - that those who recorded Jesus’ words could have embellished them, remembered differently, or added to them later. It is a logical possibility - even a probability, given how we remember things and what happens to languages. Christians today just need to come to grips with the fact that rationality, proof, air-tight evidences - all that stuff our western minds chase after so frantically will at the end of the day not be enough (even if we were ‘successful’ with it.) We so badly want to imagine that we can build a bridge where every brick is human logic and reason - no atom of faith needed anywhere in it; and then (the Christian fantasy continues…) we drag the poor atheists to this bridge where they fail to find anything unsound about it, and are forced to concede and cross over with us into the promise land. And even if that fantasy were all to come true - it would still end up being a very human ‘salvation’ instead of the Divine one. [And I must also concede that this hasn’t always been just ‘fantasy’ - there may be a good many historically who actually were ‘compelled’ by the likes of Aquinas or later luminaries like Lewis, and on the strength of human argumentation, made their first wavering steps into faith. But I would also then insist that I don’t think God leaves them there. So I don’t want to insist that things can’t at least start with the intellect. Many times they have.]
It isn’t that there aren’t answers (even good ones I think!) that can be given to your friends. But I just wouldn’t try to build your castle on Lewis’ Trilemma. We should concede from the outset that faith will be in play - and that faith will include a trust that God had a hand in how scriptures were later set down, compiled - and even canonized much later. Our willingness to look for the ongoing work of the spirit far past just Bible times and early church times is key here. We trust that the words attributed to Christ (remembered, penned, translated and paraphrased as they inevitably must be - which will necessarily make them not the exact Aramaic words he actually spoke) will nonetheless be what the Spirit intends for us to receive from Christ’s teaching. And even more importantly yet - then, the testimonies of changed lives and the effects those lives have had, both then and now are all part of a powerfull collective testimony (not proof - or even evidentiary compulsion necessarily) but just an open invitation there for all to taste and see for themselves. Our perennial temptation to nudge it over into compulsion territory seems to always be where the mischief starts. But I’m pretty sure God isn’t interested in taking prisoners.
That’s my two cents anyway.