Thank you both.

I found a similar list of programs that handle the file extension but I'm concerned over the size of the file. Opening 2GB in an image viewer seems a big ask to me. I was hoping someone had done so before and could pinpoint a particular method.

I shall play the good guy and grab a bunch of software for him to try. I doubt he'd want, or need, to pay me to try them one by one. He's also email the gov. to ask the same question, but I think it unlikely he'll reach anyone who actually uses those files.