My client has been told by his previous developer that he should never update his wordpress.

Last update was done in 2015.

Fuck.

May be he had some custom code in the core files.

Try to update it in staging environment and just revert it if you have Git

Made a backup just to be sure. But the site is 5 static pages with a default theme.

Everything is working great now.

Backup was a bit hard, he was using some other server than localhost. It was connecting to mysql.theircompany and he had another 5 wp sites in the same database. Much secure.

So I migrated my clients tables the fuck out of there.

@MisterArie commendable job.

Glad things work out fine.