Does blockchain.info show balances for addresses that are in cold storage?
... is there any way for me in another country to confirm that what my colleague views is actually accurate and correct?
Since they use Bitcoin Core, yes, there is a way to verify that they hold the addresses that they claim. Have them sign a message with each address that they claim to have the holdings on, using Bitcoin Core you can verify that they indeed have those addresses and check them on blockchain.info to find the current balance. This feature has been in Bitcoin Core for a long time.
Only works in Bitcoin Core currently for addresses starting with a '1' (not Segwit addresses starting with a '3' and not Bech32 addresses starting with 'bc1' - the developers are aware of this and I will remind them shortly.)
In Bitcoin Core, your transaction opposite goes to File -> Sign Message and signs any message with one of the holding addresses. Copy the message, address and signature and send to you via probably plain text format email is the easiest. Repeat for each additional address holding the balance of BTC that they are offering to sell.
In Bitcoin Core, you go to File -> Verify Message and key the details provided EXACTLY - spaces, new lines and all characters must be an EXACT match. Click on verify and voilà.
I prefer the form of signed message as follows (don't key the top and bottom bar rows for the message, just the contents and you can check this yourself, the bottom row is the signature). I like to key the address used for verifying as a part of the message but that is not strictly necessary:
Something that I want to sign.
This contains all of the compact information necessary to verify the message.
Example of verified message: