While it's good to provide an apology for the truth, I suspect you'd be wasting your time, since you're not going to convince a Mormon on this, not this way - it's too central a part of their thinking (that the "real" Apostolic Church evaporated due to the corruption of the Church, it's early apostacy, and ressurection under Joseph Smith.)
You'd be better saying to the Mormon something like this...
"While it is obvious you have doubts about how reflective the Council of Nicea was of Apostolic Christianity, in all honesty I do not see how you can have such doubts while accepting Mormon doctrine - since it's inherent polytheism, strange teaching on personal deification (the belief that "God" was once a man like us, and being a good Mormon will allow one to become a "God" eventually in the same way), temple cult, etc. have absolutely no documentary evidence in any of the writings (orthodox or heretical) of the pre-Nicean period. Indeed, I find it odd that you have such a high level of skepticism toward one party, yet almost complete naivity toward the claims of another."
Thus, put them on the defensive - their psyche is such, unfortunately, that arguing FOR Orthodoxy, without undermining their blasphemies, will probably not work.