None of us is a mind-reader
Heidi and her new boyfriend had already discussed that they weren't seeing other people. So after her initial shock dissipated, she took a very controlled approach.
"I saw you're on Bumble," she said. "Are we still doing that?”
His response? "No, we're not." Apparently he just hadn't gotten around to deleting his dating apps; and his verbal agreement was the final confirmation Heidi needed. In her mind, the case was closed -- and the two are still happily together.
By being so direct, Heidi saved herself the torture of spending the rest of her vacation in her own head, trying to figure out what was in his. Of course, my professional advice from the get-go would have been to roll the dating app talk into the "are we exclusive" convo so all bases were covered before these two went away together. Doing so would have prevented Heidi from feeling blindsided -- either because her boyfriend would have already deleted the app(s) in question, or because she would have had a clear sense of where they were in their relationship so she could deal with the inconsistency appropriately and with clear boundaries.