Considering that she is an ex, I would consider it stalking and worthy of at least asking the police about, even if he chooses not to file a report at this time. I'd even be wary if she was still a partner, but under the 'controlling' heading (which is frequently a lead-in to abuse of some sort). About the only time I'd see it as something excusable would be if it was a parent keeping tabs on a minor child.
EDIT: With a little google-work, I found that the only way she would be able to do this would be to have installed something on his actual phone. She may have tricked him into downloading something, claiming it was a picture or such-like. He may be able to take the phone to where-ever he bought it and have them clear off any non-factory apps.