While I've never had severe acne, I have had and still get the occasional blemish. I have had success shaving over some without cutting, and I've sliced a few. To be perfectly honest, I would recommend holding off using the straight until it got to be a little more under control. However, I would start wetshaving with a brush and quality soap and maybe using a bic sensitive or Trac II cartridge razor (simply because they are easier to get the angles right). I'd recommend following the shaves with really cold water rinses and witch hazel as well because it's a great toning astringent.

Traditional wetshaving is an exfoliating endeavor, and it may actually cause more acne problems in the short term before improving the situation (at least that was my experience in my late 20s).

Good luck to your friend