If I understand you correctly, you are not a partial owner of the condo. However, your friend signed a document which states that you are to receive $50,000 from the sale of the condo. You are wondering whether you can file a lien on the property.
I'm sorry to say that the answer is no, you cannot file a lien on the property. Liens are only granted in certain specific situations, such as if the owner expressly grants you a lien, or if you obtain a money judgment against the owner.
Based on what you wrote, your friend did not expressly grant you a lien. Accordingly, a court would assume that your friend did not want you to have a lien. Having said that, if the agreement is an enforceable contract, and the $50,000 is not paid when it's supposed to be paid, then you can sue your friend (or her estate if she is dead) and try to get a money judgment for $50,000. If you win, then the judgment would become a lien on your friend's real estate (if she has any at that point).
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.