I'm stuck for an expression that describes this example, which is similar to a "self-fulfilling prophecy" but is more complicated:
Someone claims, falsely, to be a dentist. Someone who likes that person, but doesn't realize he's not really a dentist, claims, falsely, that the guy did a fantastic job on his implants. He's sure his friend would do a fantastic job, he just needs a "helping hand" to get started.
So it's not a self-fulfilling prophecy (a prediction that causes itself to become true), it's a little white lie that unwittingly advances a deception. Self-serving (one's own interests, often in disregard of the truth) is close but in this case it looks like there's no "self" in the action.