Hi, My name is XXXXX XXXXX I’m happy to assist you with your question today.
There is probably a grieveable action here but there are two issue. As a just cause employee union workers have the right to industrial due process. One of the elements of due process is fair punishment both in terms of the punishment meeting the crime and the employee being given punishment roughly equal to what other employees get for the same act.
There are problems with this case. First you may have an issue of timeliness of filing this grievance. Being that it was relatively a long time ago. Second, its not clear from your question that the suspension was more harsh than previous discipline given to other employees with similar behavior. You are attempting to make an argument that it is unfair in comparison to punishment that occurred after this, which doesn't establish an argument for unfair treatment
...unless you are trying to make the argument that the latter punishment (or lack there of) should be increased.
Overall though, you may very well want to file the grievance anyway. You have a duty to represent employees. This probably isn't a case that could be won in arbitration because of the above reasons...timeliness and lack of past incidences to show disparate treatment of this particular employee. But, filing the grievance at least gives the union/employer a chance to negotiate a resolution...maybe even reduce the suspension. It also lets the employee the union is looking out for him, even if not successful. And it lets the employer know the union is monitoring the actions and to expect a dispute
if further similar actions occur.
I believe this answers your question. However, if you need clarification or have follow-up questions regarding this matter, I will be happy to continue our conversation – simply reply to this answer. If you are otherwise satisfied with my response, please leave a positive rating as it is the only way I am able to get credit for my answers. Thank you, XXXXX XXXXX wish you all the best with this matter.