I would try the genius bar and ask real nicely, maybe even offer to pay. I had a key come up missing once on my 2008 non-unibody pro and went into the store and talked to the genius. He went in back and came out with the key replacement, no questions asked. It is worth a try, and yes I did have Apple Care.
Unfortunately when it comes to the rubber tip, or the "nipple" as you called it, that cannot be replaced. It is glued to the keyboard and its nearly impossible to glue back on again. Usually it requires that you replace the entire keyboard. Sorry to be the bearer of bad news, but I have yet to see someone successfully replace the rubber part of a key on a keyboard and I've been replacing keyboards for quite some time.
As Kyzelios said, if the keyboard is from the same generation of MBP, then it should work. I don't know which MBP models you're comparing so I can't give you any more definitive advice than that. Are these both the silver-key MacBook Pros?