The idea that exceedingly expensive diamond (and ONLY diamond) engagement and wedding rings are a MANDATORY custom is a fairly recent invention of De Beers.
I have a female friend and when I asked her if, say, I was asking someone to marry me and her favorite color was blue, and I got her a huge and expensive sapphire engagement ring instead of diamond, she reacted like I was suggesting using a plastic ring from a crackerjack box.
Your friend is more than a little bit behind the times. In recent years, it has become quite popular, in America, to have a diamond and another stone, or even another stone...such as a ruby or a sapphire as an engagement ring. This trend actually began when the then Lady Di got a HUGE sphere surrounded by very small diamonds, as an engagement ring. In England, it has been far more usual for an engagement ring to be a regular looking ring and NOT a diamond solitaire.
Trends come and go. And a BIG, expensive diamond engagement ring is NOT something all that "new". The 1930's saw rings that were glass doorknobs! In Paris, in the 1920s, some women gad diamond engagement rings with the stone being the size of a nickle. What is NEW, is that sometimes girls demand honking big stones, from guys who really can't afford them. And what is considered to be BIG in one area of the country, is not considered to be all that big in another.
If you'd like the titles of some reference material, I can give it you via FREEPmail. :-)