If the client's website is getting "Connection Not Secure" it is most likely that they don't have SSL Certificate or they have an incorrect one that doesn't resolve to the correct server. To check, you can use this tool:

https://www.sslshopper.com/ssl-checker.html

Resolution: You can offer the client an SSL Certificate for $75 setup fee + $99 per/yr. Please contact Jason or support to set this up.