What to do if reviewers reject a paper without understanding the content?
F'x has ably covered one possible reason: that it's not you, it's them.

I'm going to cover the other side of things. That is, starting from the assumption that the editor has made a good decision.

The authors should consider rewriting the abstract and introduction. If two peer reviewers didn't understand the paper, the paper may just need a savage reworking.

The authors may also wish to try working with a freelance development editor.

Finally, the authors might want to bring on board a co-author who's been frequently published: I expect that many decent-sized, decent-quality departments have at least one person whose quality of writing leads to get manuscripts getting accepted first time, pretty much every time.

And then submit to a different journal. I don't know the field, but I'm willing to speculate that there are a few reputable journals where the article could be published.