It's a very subtle difference and results with Google suggest fortsatt is used a lot more often than fremdeles, so maybe the difference is disappearing in everyday language? However, the difference might become clearer if you translate it into German, in case someone is learning that too. Fortsatt is "weiterhin", it is oriented more towards the future, as in today and continuing in the future (fortsatt, common root with "forsetzen" in German?) while fremdeles is "immer noch", it is looking at the past, you could imagine someone saying "Is she still eating with us after what happened last time?" Not sure that's how Norwegians use it, but that's how I would interpret it.