>>Can you clarify what you mean by 'he cannot see the javascript message' ?
>
>This is the same site as here. Sometimes, you get a javascript message confirming the success of an operation or a confirmation the user has to give. This is from the alert() function. This does not show on his phone.

From the Useragent it looks like this is the Android Chrome browser. There's an option under 'Site Settings' to allow or block pop-ups. Maybe they're blocked ?