Working with a friend often seems like a great idea to many people, they think that "2 heads are better than 1".
They also think more content = more traffic = more money.
If you are doing this as a hobby, or for fun, sure go ahead. But it looks like this is more of a business venture to you, and i strongly recommend against it. Sharing Adsense revenue is not only against the T&C guidelines but is also just generally stupid, why would you want to give away 50% of your potential income?
In general, businesses with friends and family members end up being flushed down the shitter. There are obvious exceptions to this, but one disagreement/fallout and your business will go tits up. Who gets to make the final decision? Who has more control?
If its his Adsense account then dont bother, if its yours, dont bother sharing profits, just pay him for each article he writes and then forget about it. I am assuming it is his Adsense account, so i would really recommend avoiding sharing "profits".
Besides, if you are just starting, profits are a very long way away, and you should focus more on doing this for experience, as you mentioned in your post.
If you are going to write for his blog, either do it for free in return for the experience, or get someone to pay you upfront for your content.
