Whatever documentation you have for the late fees would be helpful.  If you have a ledger that you keep on a regular basis, showing the date and amount of rent payments you receive that you make a practice of keeping current, you can use that.

You are facing the possibility of being sued for twice what the tenant is entitled to, because the deposit return with the itemized damages was supposed to be done in three weeks, not four, under the law.  You will need to convince a judge that you have acted in good faith, to keep this under control.