Without the HTML source code surrounding this button, it is impossible to find the reason for this behavior. Besides the fact that there are different browsers each with their own additions that also have their own settings for each, the "protection" of the anti virus software all affects the way a web page is displayed.
There doesn't seem to be anything wrong with the web page code because you don't have the problem. You don't want to let the other person throw away their browser and anti-virus settings. Sometimes it is easy to solve by using a different browser for that website. If that has no effect, you could consider using free portable apps to look at the Firefox and Chrome browsers out-of-the-box to determine whether the used settings and anti-virus settings are the correct ones cause of the problem.