It is not a locution and is perfectly understood with friend, of the (contraction), soul ("anima"). And on top of that the phrase is put as if it could be a synonym in the debatable friend of the heart (which for me is not a locution either) and mentioned in 'inseparables' (which is in the plural, that's why I don't link it either) and in 'friends of the soul' (which obviously should not be even as a dictionary entry). It also wanted to be synonymous in chochi, but it was cut word by word (which would not be bad, if it were in the definition, but it ended like this because the site sometimes cuts the synonyms also by the spaces).