I grew up hearing people say that you should spend a month or two worth of wages on engagement rings. Many rings I've found are about $1,000-1,500, but of course, you could go up to tens of thousands worth of dollars on an engagement ring.
Personally, I think you should consider what would be right for your situation. The couple should discuss whether marriage is their best bet or their alternatives. What about a simple commitment ceremony? Find your own way to be together. No one is forcing you to follow tradition, let alone buy a diamond.
Try a different gem! Find a ring that matches the couple.
There are many independent sellers on Etsy who create beautiful, unique and often very reasonably priced engagement rings for a few hundred dollars.