"should" because why, precisely?
If you mean, because their pictures won't meet your standards (or some mythical set of standards), they don't care about those standards - if the pictures make them happy that's all that really counts.
If it's that you're worried that they'll be disappointed, afterwards, with the resulting pictures from their phone, I can understand, and somewhat identify, with that logic. But, again, they aren't likely going from no camera at all to buying (say) an iPhone 7 Plus and heading out the door on their around-the-world honeymoon the next day. More likely they have experience with their phone's camera and know roughly what to expect from it, and whether they're generally happy with such results, before they take it on a trip. Yes, they'll be missing out on the better pictures they might get from more specialized equipment, but they likely don't care.
The camera I took on my honeymoon, way back when, was a Canon S500 Elph - 5 megapixels with a 3x optical zoom (it was the nicest camera I'd ever owned, at that point). Took a ton of pictures, and some of them are really fabulous, and they stand up quite well today. Didn't use the zoom much. And I think most of those pictures could have been taken just as well with the iPhone 7 Plus today, and in many cases would end up looking better than the results I got back then. (On a related note, my avatar is one of my personal favorites from that trip, an enigmatic self-portrait - my shadow on the floor of Abbey Fontevraud in France, backlit by a really lovely stained glass window - it loses a lot in the compression to avatar size, but it brings back fond memories whenever I see it. I was having fun messing with the exposure and such on that trip, specifically while shooting stained glass, trying to get it to look as sublime on "film" as it did in real life, all while only having the camera's 1.5" LCD for judging the results, until we got home. I suspect that would give even the latest phones some trouble, getting great images in those conditions.)