Uptown's got it right!
If they can really afford it, and can still afford it in 3-5 years w/o having to refinance in the mean time, then I'd say let them go for it! It's a free country.
Your situation reminds me of the John Carpenter movie They Live from 1988, where Roddy Piper's character finds a pair of special sunglasses which allow him to see the 'truth' (that there are aliens living among us).
At one point in the story, he tries to convince his friend to try on the sunglasses, and the friend doesn't want to. This leads to a fistfight, with Roddy finally winning and putting the glasses on his friend's face by force, revealing the truth to him. A clever twist on the rose-colored glasses analogy.
Are you willing to go to this length, and (more importantly), do you have the pair of special glasses handy?