My Website (Wordpress) Loads Bad when (www.) and it's normal when acessed without (www.)

https://www.excelintercambio.com/ (With www. loads all wrong) https://excelintercambio.com/ (Without www. looks fine)

The website it's not mine, I've been doing some SEO work for a client and his website turned into this out of nowhere. Of course, the heat is on me and the client is asking like crazy to me to resolve. I'm not at all expert with wordpress and was just editing texts for his website, so I'm pretty sure that I didn't messed up anything.

However I need to fix this.

Can you guys help me with? It's suposed to look be acessable as www.