I am going to include an attorney's link here which addresses your question. I would suggest you read this and its comments. I would also recommend the association consider consulting an attorney to clarify your position to determine what steps, if any, the association should take. Alternatively, if you have a management company they might be able to help you.
The following is from Naheed Amdani's (attorney) blog:
"The new buyer is only responsible for up to six months of unpaid assessments. Unfortunately, if more than that accrues before the bank forecloses and takes over, associations often end up eating the loss. It is my understanding that before the six-month rule was instituted, associations were not entitled to any past due assessments at all."
"Special assessments that are unpaid by unit owners when the unit is foreclosed usually end up being the responsibility of the bank who owns the property after the foreclosure is complete. Whether or not it gets passed on to the new owner of the unit depends on what the new owner and the bank negotiate prior to the new owner's closing. That doesn't mean, of course, that the condominium association won't go after you for the funds!"
Be careful with the language in the previous paragraph. In Illinois there are several months when a property is in pre-foreclosure. The bank does not own the property during this time and as I understand it they would not be responsible for the special assessment when they foreclose. Again, I suggest speaking with an attorney regarding your specific issue.