You should call BT on 0330 123 4150 as soon as possible to secure the account.
This is a customer-to-customer help forum so no-one here will be able to help with the account.
The sages and experts might post a few tips later but your first contact should be BT on the number above.
Unfortunately, if his address book has been stolen it can't be 'unstolen' and the hacker will have access to it in perpetuity and free to send emails purportedly coming from him.
Make sure any passwords are changed and his contacts are warned to beware of any suspicious emails purportedly coming from him.
Have a read through this link and sub links.
How can I tell if my email account is no longer secure? | BT Help
Do not change the password until you have checked there are no "forwarding" email addresses and/or "rules" set up.
He should also contact any financial institutions such as his bank and advise them of the hack so that they can monitor his account. He should also change any password to the banks etc but only after he has checked the above and changed the BTMail password.
Does he know how the account was hacked?