someone has changed my brother's password and secret question, so he can't use his email any more. Is there something you can do about this?
The person sometimes comes online with his mail adress, can this cause any problem for him?

There is nothing we can do for you. The only thing your brother could try, is to send an email to Hotmail Support, giving as much details as possible, to show his account has been taken over.

If that person comes online with your brother's address, he could impersonate your brother and make it look like your brother says/does/thinks certain things, while he doesn't. That could be annoying.

You could try http://support.microsoft.com/default.aspx?pr=cntactms and http://www.hotmail.msn.com/cgi-bin/support for support from Microsoft regarding this.

be afraid be very afraid....

Try guessing the secret question maybe ull get it right.. or just let the account go and get a new account and tell everyone what happend