There was no wrong advice. He pleaded guilty. Sentencing is beyond the lawyer’s control. The lawyer did his part by negotiating with the DPP for a fine, and the DPP sought only a fine. What more do you expect the lawyer to do?
I suspect—like you—that the lawyer may have advised him to swallow his pride and serve five days swak swak khi. Of course, he wasn’t expecting a jail sentence. The DPP had already agreed to seek only a fine. It was the judge who threw a spanner in the works.