I have no brewing experience but I had a half-gallon of cider in my fridge that started fermenting after we had only drank one glass. It seems the fermentation has slowed now as the bottle is no longer bulging but I don't know where to go from here. Is it safe to drink? Should I boil it just in case? TIA
I doubt it fully fermented if sealed all this time. Being that this wasn't intentionally done with sanitation in mind I would discard it.
My concern would be that someone may have drank directly from it and introduced a host of bacteria from saliva and the fermentation didn't reach a point for the alcohol to kill off said bacteria because of the sealed bottle.
It depends where your cider came from. Apple peels, like most fruit, have wild yeast. If you pressed or juiced it from the fruit you would have likely introduced this yeast which would make it ferment. If it has been exposed to open air for a prolonged period of time it would have also likely been exposed to yeasts and bacterias in the breeze.
I'd have a smell and take a sip. If it smells and tastes alright then you should be fine. I've had apple juice that has spontaneously fermented from wild yeasts before, although the quality is pretty varied. There are cider recipes that still call for wild yeasts so you're not the first person to try it.