OK, story time:
Way back, a friend of mine in High School wanted to give his girlfriend a ring just randomly (for no occasion), so he bought a cheap one, like $10. So he's going to wrap it as a gift at home, and he needs a box. Asks his big sister, and he gets an unused proper ring box from a local jewellery store. Wraps gift, puts in girlfriend's mailbox as cute surprise, and when she opens it of course it's too small. So girlfriend takes box and ring to the jewellery store named on the box, and asks to swap it for one that fits. And the shop assistant doesn't spot it's a $10 ring, so girlfriend gets a nice silver/zirconium ring that fits, costing like $150. The day after she tells boyfriend she had to go to the store and swap it. After a few minutes he realises what's happened, but of course keeps very quiet to her about the true story, and enjoys his status as the super romantic/affectionate guy.
> Lying is a great way to start a relationship.
It sounded to me like the advice was "show her the fake diamond, so that she can see that it's visually indistinguishable from a real one" rather than "present her the fake diamond as a real one".