Why is there a distinction between friction and indexed derailleurs?
With the possible exception of the newer electromechanical shifting systems, the derailleurs themselves are not "indexed"; they move within a range according to the cable movement. The shifters control the cable movement, and are the thing that are indexed — not the derailleurs.

Most likely what people are referring to is whether the derailleurs are part of a part of a group that uses indexed shifting. But assuming the same high-low range, take-up capacity (for a rear derailleur) and cable pull ratio, a derailleur that was originally part of a friction-shifting group should be interchangeable with an indexed shifting group and vice-versa.