It's more than fine to loan a proven deck to a friend, but make sure you have some testing sessions set up. Vintage as a game wins people over on it's own. But make them pay for their own entrance fee. It'll invest them into what they are doing, but also just let them keep any winnings as it'll help get them hooked.
I'd even go so far as to gather up extra sets of Brainstorms, REBs, Lotus Petal, Yawgmoth's Will, Tinker and revised copies of Sol Ring, Demonic Tutor, Fastbond, Mana Vault, Wheel of Fortune, etc for them. With

proxies, the entrance barrier of Vintage can seem a lot less daunting if they have staples to start with if they can get a hold of other playables.
If you can gather the cards, I would guess that a Fish-type deck for a player who is good with interacting. Maybe not format-knowledge intensive stuff like Therapy and Mage, but Duress, Dark Confidant, and Goyfs are fine. In reality, there's really a deck that fits any style of player if they've niched themselves. However, playing something like R/G Beats without it actually being a considered metagame call can show just how hopeless 'fair' decks in Vintage can be.
People I've recently introduced to Vintage have enjoyed everything from Ichorid to Oath to GAT to Stax.