I learnt piano (I had a lessons as a kid) before teaching myself guitar, and I think it did help me work out basic chords, etc., in the early days, because I already knew (for example) that the C major chord consists of the 3 notes C, E and G, and once I'd figured out where those notes were on the guitar, I was able to work out where to put my fingers on the strings to get the same notes and hence play the chord.

But I don't know if it would have been any easier or more difficult the other way round, i.e. learning guitar first then keyboard. And I never will know.

And after learning guitar I found it relatively easy to learn banjo, mandolin, etc. Although I've never been able to play fiddle - that requires technique/skill that I don't seem to have :(

I would have thought that learning any instrument and getting a basic idea of music theory would help when learning another instrument, though, even if it's in a completely different family of instruments.