Reliability Of Ijma And Tawatur
You should ask your friend to present examples. There are no modifications to the part of religion (Quran and sunnah) that has been transmitted to us through ijma and tawatur. Any questions that arise on the reliability of ijma and tawatur are actually based on incomplete understanding of the concept. I think that rather than coming up with our own assumptions, it would be more beneficial should we understand the concept and try to poke holes in it.