Legal Question in Real Estate Law in Florida

You do not have a clear right to file a lien. You may have a right to record a lis pendens, which acts as a lien, but is done only with the filing of a lawsuit. However, your "lien" is going to be wiped out by the pending foreclosure.

Your only hope on the property appears to be as a junior claimant in the foreclosure process that can make claim to any excess proceeds of a foreclosure sale. This would mean that there is equity in the property beyond the value of the mortgage, which is not a given today, and that someone is actually going to buy the property at the foreclosure sale for more than the mortgage amount.

A more realiztic option to save some of your investment would be to purchase the mortgage from the foreclosing bank, or bid at the foreclosure sale and buy the property. This may be discounted from the demanded amount. Other than that, you always have a right to sue the girlfiend (and your son, unfortunately), to seek a judgment against them. Collecting an unsecured judgment requires that they have assets other than this property.