Water chemistry can affect a skimmer too. If you dose stuff your friend doesn't, feed different foods, vitamins, etc it may need an adjustment period just how sometimes a new skimmer needs. I have a skimmer by Red Sea (albeit a different one) and it is very finicky with water chemistry. If I change a filter or reactor media, feed, etc it stops foaming for a period then catches back up. Having stated that, of course all this is predicated on you having checked and made sure it isn't a mechanical issue.