Thanks. I suspected that DMARC might be involved, and Bill explains the details.
After I figured out how to reach the spam folder in gmail, the recipient found the recent messages there.
I investigated gmail filters, and it looks like I'll be able to solve the problem with those. The emails in question always have the same To:, which gmail allows filtering on, and one of the optional actions is "never send to spam". Sounds perfect. Now I just need to explain how to do this to the non-techie recipient.
Edward