Hm....Only you know what your relationship with your friend is like, but it sounds to me like if you asked him if you could use it he'd probably say yes. Whereas if you didn't ask him, but did use it, he might ask you if you had, in which case you could say yes which might piss him of since you didn't ask, or no which might may you annoyed because you'd be lying.
I think it'd be creepy to say "Can I use it?" but much less creepy to say "Dude, I've always wanted to try one of those, would you mind if I cleaned it, used it, and cleaned it again?"
Then again, maybe your friend is on vacation and doesn't want to be bothered with instructing his housesitter on how to use his personal pleasure device...maybe you could wait until he gets back, then borrow it?
If you use it, make sure you read up on what to do and what not to do (water based lube only, for example).