Specifically with finances, the right decision can usually be made if all the numbers are really understood. The most common mistake is people just really, really don't GET how interest stacks up. Is there any way you can show them exact figures of how things will look if they go their way, versus a better option (which doesn't necessarily have to be presented as "your way"
)?
A book my mother-in-law gave us when we got married which was a real eye-opener for us was called
The Complete Financial Guide for Young Couples. It's written by a pastor, but the influence that has is merely that every few pages there's a quote from the Bible on money. For the most part it's just a regular financial book, just sort of glazed with some stuff aimed to appeal to religious people. The thing that makes it great is it is chock full of real examples--"this is how much money you'd have after X years with Plan A, and Plan B, and Plan C," and such.
It changed our outlook completely, anyway, so maybe it could help your friend, if they wouldn't be too turned off by the Biblical inserts.