What if you don't have a bitcoin address associated with your account? does that mean you can't prove your ownership of the account?
For Future Reference (Prevention is better than a cure):
You can easily get one associated with your account,
https://bitcointalksearch.org/topic/stake-your-bitcoin-address-here-996318 , that is the thread that almost everyone uses to "stake" their address; which means you post an address that you and only you have the private key to with a signed message in your post for verification. Someone will then quote your post, signed message and they will make sure that your message is verifiable to prove your ownership of that address. At that point, anytime there is any question of ownership of your bitcointalk account you can easily produce a new signed message from your staked address and there will be no reasonable doubt that you are the owner. Obviously, this puts a level of pressure on keeping your private key and that particular address protected;
never lose this. It would be preferable that this address be a cold-storage address that is not at risk of being key-logged or infected by a virus being on your computer for regular usage.
If you do not stake an address your bitcointalk account will have one less measure to prove that it is in fact yours; without this verification I believe it is next to impossible to ever recover your account if it is hacked, you lose your password or get locked out for any other obscure reason (security questions do this, for example). Essentially, no you will not be able to provide
sufficient proof of ownership over your account without associating an address, signing a message and keeping it private.
For Now:
If your friend has not staked an address there, they must have used another address around the forum somewhere? If this is the case, it would probably be sufficient to sign an address they've used publicly around the forum.
He never used his bitcoin address here, he only used his MEW address.
Are they able to sign their MEW address?