I think it would be helpful to know exactly what they mean by the term “altered.” Is there a concrete example of a biblical alteration which has proven to be problematic to the practice of Christianity as a whole?
And exactly what is their point concerning authenticity? Are they suggesting that God somehow intervenes (or will intervene) in human affairs to prevent the Quran from being altered and that this is supposed to be a sign of divine assurance of its authenticity? I’m not going to bother doing the research, but I suspect one could find another religion or two, and maybe a number of cults, who could also claim that their sacred texts are free of alteration. If that is, indeed, the case, then that, of course, would raise a big question for the Muslims you’ve been speaking with.
I assume they are not simply talking about the challenges that naturally stem from translating biblical Hebrew and Greek into modern languages. After all, the same thing happens when translating any text, including the Arabic of the Quran (see, for example,
Difficulties of Quran Translation and
Some Linguistic Difficulties in Translating the Holy Quran from Arabic into English).
As a side note, I just Googled the question, “Has the Quran ever been altered?” It certainly seems that opinion is divided, even among scholars.