Spendthrift and prodigal are the correct English words, however they are not common and require an education than most commoners wouldn't understand. It's said the average American reads at a 7th or 8th grade-level, so keep your language simple. I know, this is sad that I have to tell you to dumb it down. But you can just say spender, big spender, or careless spender. -US English

A 'spendthrift' is someone who spends lots of money in a careless fashion. The word 'prodigal' can also be used for a person who spends money in a careless way.

I thought of another word, frivolous. That's like at a 10th or 11th grade reading level, but most college educated people would know what you meant. ie. He is frivolous with his money. This would be a common sentence among educated circles. Otherwise, what I said before is true; stick with simple words. http://dictionary.reference.com/browse/frivolous