Is this true for customize child theme?

My programmer say this

“when the parent theme got update only version file is updated, but no updating is done in the main code so there is no issue regarding overlap”

This is because he did some alteration in the parent theme to fit my child theme, so I told him the customization would be lost when parent theme has an update.

I’m worried it will affect my customization. Anyone could kindly advise? Thanks and appreciated.