A friend of mine says that one is typically used for people and the other one is for objects. The dictionary is no help, it gives the same translation for both. Is there a significant difference or are these two fully interchangeable? When should I use 'ruhig' / 'still'?
In this meaning, ruhig can clearly be used for people only, not for objects. Similarly, one meaning of still is guarded or reserved, as in
Again, this can only be used for people. So unfortunately there are no clear rules. If you really want to learn to use the two words correctly, I think you'll have to look at many examples and develop a feeling for the meanings. In the two Duden links above you'll find quite a few helpful examples.
But I agree with what others have said here already: don't be afraid of using the "wrong" word. People will still be able to understand you, even if they may pause and think for a second.
Meist kann man sie aber wechselseitig verwenden:
Eine andere Unterscheidung: Im Sinne von beruhigt wird nur ruhig verwendet:
Hier wird nicht still Blut gesagt. Blut stillen :) gibt es auch, aber um Verben ging es ja nicht.
Ein Unterschied zwischen Personen oder Sachen ist mir derart unbekannt, dass ich nachfragen muss, wofür man denn angeblich ruhig, und wofür still verwendet?
Bei einer Uhr mit Pendel/Zeiger würde man nur sagen, dass sie still steht, oder dass der Zug auf freier Strecke still steht. Aber ebenso steht der Marathonläufer mit Seitenstechen bei Kilometer 35 still.
Es gibt auch noch aus dem Wirtschaftsleben den stillen Teilhaber, der nicht durch einen ruhigen ersetzt werden kann. Aber ich kann es nur aufzählen, nicht begründen.
As other answers already pointed out, there are a lot of different meanings that those words encompass. Since the question does not clarify the context, what we're exactly talking about, I want to respond to the three intersecting definitions.
Before I start, I want to mention one very important fact. Two words are never fully interchangeable, otherwise one of those words will die soon. If two words have the same meaning, one is superfluously. That means, there is ever a subtle distinction between words, but, of course, in some contexts they are interchangeable without changing anything in meaning.
So, back to the words. The three meanings which those words share are:
(1) can be used for people, animals and objects. For (2) I'm not completely certain. You can use it for people and animals for sure, but I think it could be possible for objects, too. Though, I don't know any example right now. The latter one can only be used for people, and rarely for animals, too. Most times, when you use the latter one for animals, this is quite related to (2) as you will see in the examples below.
Note, often you will rephrase the sentences using still or ruhig as nouns (Stille, Ruhe), or using different words with similar meaning (e.g. leise). I just want to show a couple of sentences, using both the adjectives and the nouns.
Before I show the examples, I also will mention that there are a lot of examples, where the words ruhig or still are used figuratively, but I don't want to incorporate them into my answer.