The common approach is to use the RXTXcomm.jar but the main issue is this jar is not well maintained in maven repos. Looks like a good option is to use is http://fazecast.github.io/jSerialComm/
Your best bet is to go on the Java discussion board for this type of question.
Sounds like you have a bad apple. Customer complaints should almost never be excused by "they should know better" and these seem like actual functional errors. I would encourage you to replace this person as soon as possible. Happy to talk this through in a quick call
I found some helpful information here https://developers.google.com/adwords/scripts/ It looks like a basic understanding of JavaScript is sufficient to write your own script. There's a 17 minute video on that page that gives a good introduction to deploying a custom script. Best of luck!