I was in Lima, Arequipa, and the Andes in 2000 (pre-CPF).
Like mentioned earlier, beware elevation sickness. Be sure to drink plenty of water to stay hydrated, and even some Coke or Pepsi to help with blood sugar at the high altitude (chocolate works, too).
I'm not sure, but at the time I went, it was required to get shots for yellow-fever, and some other infections. You will want to be sure to be updated on shots and bring those records with.
Facilities like bathrooms range from modern to primitive depending where you are. Driving/being a passenger is often a white-knuckle feat. You are best to check ahead with the taxi services, etc. to see which ones are reputable. When we were there, we had a planned itinerary with a travel agency (Condor Travel) who picked us up at the airport (late arrival). As previously mentioned, the trip from the airport to the hotel was quite exciting (in a nervous way), too.
Heed all advice about pick-pockets,etc. Beware what you carry on your person in public in terms of bags, etc. If you get bumped into by anybody,even so slightly, there is a good chance you've been robbed. It would be a good idea to ask your Peruvian friend about what styles are current there and get some clothes that may help you blend in a bit better. Tourists are marked quickly and often by thieves and worse. I had a person following me trying to get at my daypack when I was there, until our guide said something to him in Spanish and the guy tailed off. I had been watching the guy the whole time, and he still followed me and looked like he was waiting for a good chance to do something.
You can usually find good deals for merchandise at markets. Be sure to negotiate on prices, and maybe do a little research ahead of time. Keep in mind that some things might be grabbed at Customs when you get back, so use your head when buying anything down there.
The airports we encountered were all outdoor, with a staircase up to the plane type of thing. Wasn't used to that.
Peru is in a seismically active area, so be aware there are earthquake possibilities. Modern facilities there should provide adequate shelter.
Overall, it is a naturally BEAUTIFUL country. Just beware areas in Lima, as it is quite sketchy in parts. Be aware of your surroundings and you should have a good time. You might also want to check the US State Dept website for travel alerts for the area ahead of time.
Have fun!
-MB