I don't think playing for money is a problem. Most of my friends that I have played poker with have not gotten upset because of it. I've even made friends through playing. One of the people I have played the most with, my step-brother (and the inspiration for my signature), is a better friend because of our poker play than if we didn't play.
It's a game, and most mature friendships can handle that. Yes, there is money involved, but that is just for keeping score. And, no one should be playing with an amount you can't lose.
With all that said, poker did strain my relationship with my brother-in-law for a little while. He wasn't supposed to be playing for money, but he decided to play anyway with money he couldn't afford to lose. My son proceeded to stack him 3-4 times in as many rounds on the table. Then my son, being 8 at the time, got bored and wandered off, leaving all the money just sitting there on the table. My brother-in-law got a little heated about that. But, I told him that no one has to play longer than they wish and if he had won all the money my son had, I wouldn't ask him to give it back. It started a pretty big fight between him and my sister, because he was unable to make back his losses. But, that's not the fault of poker. That's his own choices.