Breath of Life will work on any creature in your graveyard, regardless of whether it was previously in play. "Return" is functionally synonymous with "put."
When you're wondering what a card actually does, it's best to start with its Oracle text. This is the most authoritative take on "what the card says." You can look this up easily on Gatherer, which also has helpful rulings.
For example, Breath of Life reads:
Return target creature card from your graveyard to the battlefield.
In Magic, "returning" a card to the battlefield doesn't require that the card have been on the battlefield previously. As far as I can tell, this isn't explicitly spelled out in the comprehensive rules ("return" and "put" aren't treated as specially defined terms, unlike "tap" or "discard"), it's sorta just assumed. This isn't a minor technicality, either. It's the bedrock of an entire archetype known as Reanimator.
If you or your friends are truly skeptical, here's an account of a player using Unburial Rites (a card with identical wording to Breath of Life) in exactly this way during the deciding game of a major official tournament — in game 3, Mcclain puts a couple of cards into his graveyard on turn 3, then uses Unburial Rites to put a Thragtusk directly onto the battlefield on turn 4, before he has the 5 mana to cast it normally.