I would have always said the same thing. But my brother bought his new wife a 17K ring when they were both barely out of grad school, 200K in debt, living with my parents, and he financed over half that. I thought they were nuts, and I was secretly damn judgmental about it. But they are both ridiculously happy with their decision, and she'll have a ring for life that she loves and is stunning. Their financial situation will improve-- school debt isn't for nothing-- and this debt will be a blip in their lives.
If you know you're income is on the way up, and you have a plan, there's nothing wrong with financing a ring, unless you're uncomfortable with it. It's just not true that you can't afford it if you don't have the cash. I have completely changed my view on this.
I can't disagree more with this approach. How you manage five figures or even six determines whether you'll ever reach seven figures IMO. I know people who make about 160-180k a year for years and barely were able to make a dent in debt like that, after taxes and living expenses there isn't a lot left.
Also, trust me, there is nothing you love for life. Either they'll have to sell that ring when they hit financial troubles or within 5-7 years she'll want an upgrade or perhaps a matching car/earrings/bracelet.