Oh that's horrible, I would feel really betrayed. How did the couple respond to your explanation?
Will you confront the person who took your photographs without permission?
How bloody sneaky. I'd find it hard to forgive.
I've lent my closest friend/sort of partner my old phone for a few weeks as he's gone away and had to give his back as it was on a contract. It was an emergency, and before I gave it to him I went home, charged it, and wiped any messages I didn't want him to see, not because I thought he would necessarily look, but because I didn't want him to be hurt by anything I might have written in haste (my drafts file is legendary) or to think badly of me for my ridiculous mutterings.
I don't know if he has looked in these parts of the phone - I guess everything in the inbox is very easily seen as he will be using it as his own. I left a whole lot on there as I was pushed for time, but most of it was between me and him anyway so he'd already seen it.
My point is, you have to assume responsibility for your own property and privacy, whoever you lend something to, but that goes both ways and they have to assume they don't have permission to look at or take your stuff.
I'd be having proper words with this person.