The wallet should have those addresses in its buffer. Which means that if you click on "generate new address", the new addresses you'll get are the ones you had before. What they won't have is the label, which you can add.
Do note however that change addresses, generated when you send bitcoins, also count towards the 100 addresses buffer.
You should be able to see the coins in your balance and spend them even without re-generating the addresses.
I've never actually tried this before so reports on your progress will be welcome.
And I've heard that if you have someone's private key, then you can spend their bitcoins. I have my private key and my public keys. How do I access them?
Some of the dev versions of bitcoind can import and export keys.
Since you know the private key, the easiest thing to do is import them to a wallet at blockchain.info
They're already in his wallet, no need to import anything.