Guinea pigs need good quality long- strand hay at all times to help with their teeth and to keep their guts moving (which they need to have happening constantly). Timothy hay and/or orchard grass are two of the most common. Young pigs under 6 months can also have alfalfa hay along with the Timothy.
Pigs also need fresh veggies every day, preferably low in sugar and calcium. Green bell pepper, parsley, cilantro, romaine lettuce, etc. other things like carrots, cherry tomatoes, and kale are okay a few times a week but not daily. Fruits can be given as a treat, but not too often; piggies can't process sugar well. There is actually a very good chart here on the forums about which foods to feed and how often. I don't know how to link to it from my phone, sorry!
Yes, they also need pellets, an 1/8th of a cup per pig per day of Timothy pellets or, if they are under 6 months, alfalfa pellets.
Where are you and your friend located?
Oxbox is generally a good brand and is more commercially available. Lots of people here like KMS Hayloft (Kleen Momma's). It's a small company based out of the West Coast (US). She sells hay and pellets. Her pellets are lower in calcium than Oxbox's, which may or may not be an issue depending on the individual pig.
I have also noticed personally that one of my boars won't eat the Oxbox but gobbles up KMS like I never feed him ever.
Lastly, piggies, like humans, cannot generate vitamin C on their own. Vitamin C deficiency can lead to scurvy and death if not treated in time. Therefore it's important to make sure they have enough C every day. Pet stores will tell you to put drops in their water. Vitamin C is water soluble, though, meaning it breaks down in water so the drops are rubbish. Most of us feed our pigs Oxbox Daily C Tablets. My boys think it's a treat.
Guinea pigs are actually quite a lot of work. I consider it to be entirely worth it. But someone who doesn't even have time to research them before owning them...well, it raises concerns about whether your friend can truly care for them the way they need and deserve.
That being said everyone is extremely knowledgeable here and you can always find an answer to pretty much any question you've got!
