This probably will not break through to the person you are talking to (sorry I am not a prophet, seer and revelator and lack all answers).
But, in order to justify your own actions and position, I think it is important to explain that putting in the kind of effort the LDS Church requires means, at least for you, that it needs to live up to its claims. Otherwise, it is just not worth 3 hours or more of Sunday meetings, probing and poking worthiness interviews, holding a calling, etc. If the LDS Church is not what it was advertised, then it's just not worth all the effort playing Celestial Pursuit.
Second, whether the information is old or not does not matter. If some people have known that Joseph Smith didn't translate the alleged Book of Abraham papyrus correctly for decades. But it was new to me at one point--presumably because the LDS Church did not volunteer that information. At the same time whether the information is new or not is completely irrelevant to whether it is correct. Just because we have known about the reality of gravity for a long time doesn't make it irrelevant to physics--much less disprove its existence. Joseph Smith said he translated something--turns out he did no such thing. This demonstrates that he is not reliable. For a normal person, this would seem to have tremendous implications for one's decision whether or not to build the foundation of your life upon this guy's message for humanity.
It seems that your partner in conversation does not put much separation between his or her self and the LDS Church. So what you are saying is bypassing their brain and hitting an emotional sore spot. There may come a time when this person is in a different emotional space, or feeling different about the LDS Church when a rational conversation is possible--but don't hold your breath.
When a statement like this is made it shows how thoroughly the MormonCult has done its job of indoctrinating. Members are taught that good, meek and mild sheep do not do critical thinking on their own nor are they allowed to read material that might set the mind a spinnin'. So, to be in the presence of someone who does this evil thing of CRITICAL THINKING means that person is in the influence of Satan and must be called out.
Such a narrow box to live in, but when you are taught this day in and day out, it can be a very difficult box to break free from. Cult are not only narrow and restricting for those in them but are the same for those who interact with them. I still find myself getting frustrated at the nonesense and dangerous beliefs they hold to the point that sometimes I just want to shake them awake knowing that this will do no good but still almost feeling compelled to do it.