EDIT - Updated based on comments from @AdmSelec below and info from GlobalSign
Whilst usually when that error pops up it'll likely relate to SHA-1 hashing, in this case it appears more likely that it relates to a bug in Chromium on OSX (details here).
Whilst the screenshot displayed doesn't show the certificate itself it does show information on the issuing CA indicating that it uses SHA256 and usually a CA will not issue a SHA-1 signed certificate from a SHA-2 signed Intermediate.
General Answer for seeing SHA-1 cert errors
The specific concern here relates to the SHA-1 hashing algorithm that is in use as part of your certificate. SHA-1 was a commonly used algorithm for this in the past but it is now considered insecure, and modern browsers are starting to deprecate it (causing the warning your user is seeing), more information on the deprecation can be found here or here amongst others.
What you need to do is re-issue your certificate using an updated set of algorithms. The instructions will vary depending on the company issuing it, but there are some pointers here which appear to include a reference for RapidSSL.
I'd expect if you contact RapidSSL support they should also be able to help.