I moved my site to PHP7 and most things just stopped working - and I have a very simple site. Unless the owner actually went into the code to fix things, most of the site would've been blank pages.
For instance, PHP7 does not allow not mentioning the default value in function definition (or something along those lines, I don't remember), and you would normally not do that. Mysql calls are all changed to mysqli, at the very least.
In other words, what I'm saying is that if the owner cared enough to move his content to PHP7, he should've cared enough to make sure the content works.