yes, the same WiFi access. The name of the website: HTTP://www.dgbeheer.be/e-portaal/
Walter,
I originally suspect your WiFi may be the issue but that's not it. Here's the problem:
That (https://www.dgbeheer.be) website's SSL digital certificate has a problem. The SSL certificate name does not match the website address. The certificate is issued (and valid) for the following websites:
cloud.sitemn.gr
www.cloud.sitemn.gr
You're getting the (unsafe site) warning because of that problem. You said you didn't have a problem accessing the site on your friend's computer. Did you access the same secure (https) site or unsecure (http)? I am guessing you tried to access the unsecure site (http).
If you did access the same secure (https) site, your friend may have manually added an exception on his/her web browser and therefore there is no warning on his/her computer.
Here's how it looks on my computer when I access the site:

This is not something you can fix on your computer or iPad. It is the website owner's responsibility to correct the problem. Contact them.