My friend has these pimple like things that appear sometimes and it would have puss and it would leave scars. What to do?

Cystic acne. This could be from cystic acne. Get an appointment with a dermatologist.
Pimple like things. If these are really pimples due to acne, stopping nuts and peanut butter and chocolate and cutting down on dairy products will help prevent new ones. Wash with neutrogena acne wash and apply a bpo (benzoyl peroxide) cream or lotion -- something like clearasil or stri-dex -- on the affected areas at night. If it persists, see a dermatologist who may prescribe an oral antibiotic.