As someone who has to confront embarrassing financial issues with people on a regular basis, I find it easiest to just rip the bandaid off.
I don't recommend assuming it's an error (it implies a judgement that late utility bills reflect poorly on the person and therefore you don't believe it's possible). I don't recommend making any assumptions at all. Pretend you're a robot.
1) Present the problem factually and concisely without assumptions or commentary: "This notice was on the door."
2) Inquire about necessary action on your part: "Is there anything I need to do about this notice?"
3) If necessary, offer brief words of empathy to end any awkward exposition on their part, "Don't be embarrassed, this happens to a lot of people."
4) Re-orient the conversation back to a solution: "You can probably call them and pay it over the phone from where you are."
A photo and a text saying, "Hey, this was on the door this morning. Is there anything I need to do?"
The answer will probably be no, unless they need to Western Union you some cash to pay it.
Done.
Also - you're not ruining their vacation. You are providing information so that they can solve a problem promptly. Chances are that this isn't even much of a surprise.