I started getting strange messages from a friend that I felt were inappropriate from him. Not [*****] but stuff that I know he wouldn't be associated with.

Upon investigation, everyone in his address book were getting the same messages.

He asked my advice in correcting the invasion of his privacy.

He ran MS Essentials Security Scan and it was negative.

Like me, he's an old fossil and even less computer savvy.

His ISP is AT&T. Please suggest what he should do to regain control of his address book.

TIA