It's been so hot, I haven't even allowed my boars or the two guest sows to go outside. I'm not a fan of keeping guinea pigs outdoors in any case. Someone on another forum I occasionally frequent had her two dismembered by foxes last night.
Have you got a room indoors where you could keep the pig, secure from the cats? Cut handfuls of grass and give parsley (preferably flat leaf and not a whole lot) and as much cucumber and celery as the pig can eat, plus plenty of hay. Should always have constant source of hay and a dripper bottle and/or bowl of water.
If the hog isn't eating or drinking on its own, that's gonna be your job. Get some syringes or clean dropper thingies like you get in medicine bottles sorts of things, wrap and syringe in water (preferably though give rehydration solution - like Dioralyte, mixed as directed on the packet,) and the crushed up soaked green pellets from the dry pig food, made into a sort of porridge (Ideally, use a 2 ml plastic syringe with the pointy bit at the end cut off and draw it up/squish it up in that and plunge it into the mouth.
Pigs are very sensitive creatures and if it has got hot, you might find that forbidding it won't make a difference. But, it's worth a go getting it somewhere cooler, preferably indoors and safe. And, to be honest, to improve the chances, get it quick to a cavy savvy vet. They are a bit like hens teeth, but let me know roughly where you are and I'll see what I can find.
Lots of info here
http://www.guinealynx.info
That's the immediate priority, then slap the shit out of your friend when they return for not telling you what you need to do, then give them a slap from me for keeping the pig on its own unless there's a damned good reason for it because pigs hate being on their own.
Hey, good luck!