First, academia is based around the search for and discovery of truth. This often requires changing positions and conclusions to fit new observations, no matter how uncomfortable or inconvenient that may be. That should be your guiding principle in everything you do as an academic.
To answer your first question, if you had submitted a research paper that drew conclusions based on data you later learned to be faulty, would you submit a retraction? I would hope so, as you now know the conclusion to be unsupported by the data, and to continue to allow people to make decisions as if it were would be unethical. You have a duty to correct that misinformation.
As for your second question, it is irrelevant. You do not have a responsibility to ensure the success of her academic or professional career, only her instruction and the development as a student. The academic dishonesty this student has shown in cheating ultimately calls into question their entire body of work (falsus in uno, falsus in omnibus), including that which you based her recommendation on. Allowing the institution they have been admitted to to operate under the assumption that they have legitimately demonstrated themselves academically and should be admitted rewards that dishonesty and will only serve to deny them accountability and stunt their personal growth. Your student is the one who engaged in unethical behavior, and the consequences are theirs to deal with. They are responsible for their own success, academically and professionally, and if this negatively impacts this, it is of their own doing, not yours.
Your desire to be clear and honest with your colleagues is exemplary and the correct course for many reasons. If you were in their position, you would expect the same candidness and truthfulness. Your reputation is also at stake. If they later observed such behavior from this student, they may call your judgement into question. If they were to learn that you knew of this behavior and maintained your positive recommendation at their expense, I would expect them to question your judgement as well as your character. Lastly, whatever program this student was admitted to likely has limited room, and their admission comes at another student's expense. Failing to inform your colleagues about this behavior and inform their decision making may potentially deny a more deserving student this opportunity.
As for your actions moving forward, the best course of action in my opinion is to contact the university she has been admitted to and explain the situation, as well as explain your position and actions to the student. If you still believe they should be admitted to the institution and the behavior you have recently learned about is an aberration that is not indicative of their entire body of work or their character, then let them know. You can maintain a positive recommendation while still being frank and honest. If you no longer believe this student should be admitted into the university based on their actions, then change your recommendation. The pursuit of Knowledge never benefits from the obfuscation of the truth to fit personal or political agendas, and taking a course of action based on the convenience of the ramifications does no one any good in the long run. In short, be truthful because that is where your ultimate duty lies.