As others have mentioned, if the competitor is bidding on your client's brand name then it may fall under Google's trademark policy and you can file a complaint with Google.

If they're bidding on generic keywords however then the only thing to do is ensure that your quality score is as high as possible, which will reduce your ad cost and push you above your competitor whilst costing you less per click.

Most of the copycats that I've come across aren't very good when it comes to campaign optimization, meaning that most of the times it's fairly easy to achieve a much higher quality score than they'll ever see.

Make sure that your ad copy matches your keywords, that your ad copy + keywords match the content on your site, that you're linking people to pages that have good content (as opposed to "naked" landing pages), and you'll outrank your competitors shortly.