Hi candis. I posted your issue on the Proboards Support forum. Here is their response.
I'm guessing this user is coming from an IP that likely has a bad reputation. This isn't entirely uncommon and the security check should eventually stop appearing after a while when it no longer deems the IP suspicious.
If the user is browsing from a private home computer then I would recommend not logging out every single time just to reduce the amount of times you actually must perform the security check."