I am not a dev. Sounds like you need to go on github.com/megamek/megameklab and create a bug report with the log file and screenshots. I asked around about not being able to run MML on mac for this release but there needs to be a whole lot more information provided. It might be your setup.
It could be incompatible java version, permissions not set right, gatekeeper issues, paths, etc
Since MegaMekLab isn’t signed (I don’t think anyways), Gatekeeper throws a fit unless you open apps a certain way (right click and open)
Would need way more info.
Is he trying to launch from HQ, or stand alone.
Is he trying to launch the jar file,. Or trying to use the Mac app
Personally, I don't use the Mac app, and always down load the "Linux" version
Then launch the jar from the command line.