I work with a group of geologists. We have one manager, but the rest of us (seven total) perform our job duties as equals. One geologist, who mentored me when I began this job, has made great contributions to the team. This geologist got the team up and running in more advanced geologic software, which has been a great boon to our group. Additionally, this geologist has lots of good knowledge and skills.
However, this same geologist has on several occasions looked over my shoulder as I worked, and offered geologic opinions that were wrong (you'll have to take my word on it, I had been working on the issues for hours, days, and in some cases months). In one instance they spent over an hour trying to inadvertently mislead me, which I did not appreciate at all as we are very busy at all times. This geologist has never admitted they were wrong, and often brings up one of the cases as if I had been mistaken. I have determined this geologists main professional flaw is they are too confident, without the geologic background to even support overconfidence.
Unfortunately, in our job there is not much time for peer review, so much of our work goes on without any. After several of these cases of being challenged by this geologist, knowing they are 100% incorrect in what they are asserting, it got to the point that I no longer wished to work with them regarding geologic work. Of course, on using software and data management issues, they are very capable and important to work with. But when it comes to geology, I had lost confidence that I could get a good opinion from them. This overconfidence shows up in other areas - it seems that when this geologist gets an idea in their head, it becomes the most important topic and must be addressed. They had expressed concern about another state likely causing sinkholes in our own when there was absolutely no evidence for such - the only connection is that sinkholes occur in the same geologic areas that extend between the two states.
This geologist I am concerned about has entered somewhat of a team-lead position in our group. There is certainly justification for this, considering their aptitude for using our main geologic software. But knowing that they are leading projects of a geologic nature, and making key decisions in some areas related to the geology, with hardly any peer review of their work, makes me uncomfortable.
I don't know what good expressing this to my manager would be. The performance of this colleague does not affect me much as I will leaving this group to start a new position soon. But recently my manager expressed that they were going to start a more official team-lead position in the group, and from the context of things, I know that it would be this particular geologist of concern whom would be promoted to it. I would honestly be irritated to know they were in a position I needed to report to, and I if I was to continue to work here, I would be more likely to express this to my manager. Should I express it anyway in concern for my other colleagues?
Would it do any good to express my concern to my manager? Or to be professional, should I simply not say anything and let it be? Should I discuss this with another colleague first? I don't want to make this particular colleague look bad, but as a professional geologist, I feel some responsibility to raise awareness about some particulars of their performance, especially when my manager is viewing them as fitting for a team lead position.