I know what you're saying, but if you were offered a choice between buying a car for £20,000 and buying the identical vehicle for £60,000 then choosing the latter would undoubtedly be a waste of money even though you'd be buying a car. Most people would probably still think it wasteful to choose option two if the rip-off garage said they'd throw in a tank of fuel and some mats for your £40,000.
For the OP, her husband's car choice seems like a waste in much the same way. He gets a product that fulfils a need that could have been fulfilled much more cheaply. Yes, he gets the additional prestige of a premium car brand, but she doesn't value that any more than she would a tank of fuel and a set of mats.
I'm not saying it's morally wrong to drive a Merc, and I don't drive a Dacia myself. I'm just saying that whether it's a "waste of money" to buy a more expensive item is subjective. Some people place no value on the cachet of a premium make, and it can reasonably be said that a more expensive car with no demonstrable practical advantages is a waste of money to those people.
Others choose to buy flashy cars and that's fine. You could say it's not a waste of money because they find value in it, or you could say that it's fine for them to waste money in that way because they enjoy it, just as some people enjoy "wasting" money on designer clothes, single malt, Champagne and spa treatments.