A lot of things could happen to cause the toilet to siphon out. Back when I did service calls a lot of times there was something stuck in the trapway. Such as wad of TP, or feminine hygiene product. Might recommend the owner to purchase a Rigid K-3 Toilet Auger. $25 or so at HD. And try to see if anything gets stuck on the end of it, such as paper. (I'm sure everybody has a plunger, but an auger is handy as well, and I would never recommend anything else but Rigid K-3 with a bulb head. It's great for cleaning toilet trap ways.) Blockage in their main drain lines could cause an extra siphoning, if not vented properly/adequately Inadequate venting could also cause this. There maybe just enough venting for that toilet when it's flushed, but if there is a surge in the main sewer line around their house, this could cause traps in that house to siphon out. Never rule out the phenomenon of the wind playing a role. ( I had always doubted that theory, until a few years ago, when it was the only logical explanation, but that is a long, long story) Frank