The 16-year-old Australian singer - who supported the Canadian popstar on tour recently - is happy the on/off couple have reunited after breaking up in December.
He told HollywoodLife.com: ''It's good to see that. She's always been a great influence on him.
''She's amazing every time I've met her when I'm with him. She's a super sweet girl. I'm happy for them.''
Selena, 20, reconciled with the 19-year-old 'Boyfriend' singer last week after flying to see him in Norway.
Although friends of the singer are reportedly unimpressed she is back together with him, sources said she only realised how much she missed him when she had time off alone recently.
Cody previously defended Justin's bad behaviour earlier this year, after he endured a nightmare week in London where he argued with the paparazzi, made fans wait two hours for him to perform and collapsed on stage.
Cody said at the time: ''He's a really good guy.
''Stuff that's really not that bad is made a big deal. People like to do that about everyone. You just gotta deal with it. [You have to] be as sensible as you can. At the end of the day, we're teenagers!
''We went to the Eiffel Tower and stuff at 1 a.m. so we didn't have anyone bother us. It was a lot of fun. He's a great guy to tour with. I had an awesome time in Europe.''