You could always tell your client to install a child theme plugin.
But as a developer working on someone's site, why not change your process to ALWAYS make a child theme? Unless you're making a 'for them' custom theme, why would you not follow best standards and practices? YOU know you should do it :) So do it from the get go.
Think of it from a practical perspective.
These are people who pay you. What's going to cause you the least amount of 3am phone calls?
1) Make a child theme
2) DISABLE the in-admin editor so they can't break their plugins and themes
3) Have an SLA about response times and costs
Establishing a professional relationship with people who hire you is as much for the protection of your sanity as it is for making money :)
If you're not charging people and they're not paying you for this kind of thing, then you don't have a professional business relationship, you have an abusive one.
