If it's been over 2 yrs since job was completed, can you still file a lien if customer is refusing to pay in full?

My husband owns and operators his own private construction business. He completed a job approx 2 yrs ago and the customer continues to refuse to pay in full. I am not sure of how to explain all the in's and out's, but Im sure there was a falling out or disagreement of some sort between them both which she has only paid approx half of what was owed (total owed $21, 000). My husband let it go for so long only bc of how busy he was in general with work, thinking she would pay at some point. She is a realtor and her husband is a doctor.