Security Issue with fundraising

Hey there,
My client is using your fundraiser plugin on his website. When he made the donation, he received a message saying that it (donation) was viewable over the Internet.

What does this mean and what can I do to prevent the issue and future messages like this?

I understand that it can easily be prevented by entering the desired page I want to "hide" in my robots.txt file.

However, what page is that and how do I do that?

Sincerely,
Mika