Former employer accessed personal Gmail account & forwarded personal emails after I quit

Hi! I'm posting this question for my boyfriend. He spent the last few years working for two music venues in Denver, CO. He was unhappy with the lack of advancement so he formally submitted his resignation. His last day of work came and went about a week ago and today he went to his Gmail account to follow up on some job inquiry emails he had sent. Checked his sent folder and found that his former employer had forwarded all of them to herself. Is this illegal? He must have mistakenly saved his password on one of their machines, but I don't think that really counts as consent to have his personal email accessed and sent out.