It is indeed risky. You need to have complete control over the content where your ads are being displayed in order to prevent getting banned from Adsense. You might also run into problems if the person you "share" your account with decides to go on a clicking spree in order to make more money. Taxes could also become an issue since you will be responsible for any taxes payable on the revenue your account generates. Why can't this person apply for his own account?
Yeah... you can get your own adsense account even if you just have a blog. Just tell him to get his own account.
Think a little. If the dude can't get his own account...maybe he's been banned or Google doesn't like his site and denied his application. Would it be OK if I 'shared' my bank account with some drunk stranger??!!!
That's right.. why don't you go make your own account... You might be banned and risk someone else's account to get banned.
It is risky but some free host providers take the risk (I saw a free hosting site that placed adsense ads on all the sites that they hosted, that's more of a risk because there are so many sites that are being maintained by people you don't know). If you trust your partner and you know which site(s) they're placing your ads in (so you can review it from time to time), then it should be less risky..