Are they using the newest versions of all the plugins? It’s working just fine on my development/test system, and the Customizer is a very simple extension with only a few small files that doesn’t do a whole lot other than filter/change the text and create a simple admin interface for the settings. Only 3 files with very little code overall. Nothing that is creating any databases or anything that should cause a server error. Worst case, with version mis-match there might be some PHP warnings/notices if it’s trying to filter some text strings that don’t exist in the settings array because it’s out of date.
Are they trying to set it up on a Multi-Site install? I don’t think the member directory extensions have been setup/tested for multi-site. If that’s the case, let me know and I’ll take a look. Usually just need to change something in the activation, or dependency check (to see if main plugin is installed), for it to work correctly on multi-site.
It it’s not either of those 2 things, then I would need a lot more details, or possibly a temporary admin login (email me directly for that, don’t post it here) to take a look at what’s happening.