The best beaches are all within driving distance of Lima. If your friends are thinking of splitting their journey halfway, they aren't going to find anything in the way of proper beach towns/resorts. The last major resort I can think of heading south is at Paracas, which I don't think covers even a third of the journey to Arequipa.
There are a few places like Puerto de Lomas and Marcona that have beaches, but they're a bit off the beaten track and I don't know how difficult it is to reach them if you're travelling south with a major bus company.
Perhaps someone knows the road better than me or can recommend some hidden gems, but if your friends are after beaches, I think it would probably be a better idea to take a plane to Arequipa and, once there, go to a beach resort like Mejia, or Punta de Bombon.http://www.todoarequipa.com/fotos/islay/punta-bombon/http://www.maplandia.com/peru/arequipa/ ... as/hotels/