Hi, I'd like to help you with your question.
It hurts to be treated in such a way by your employer, particularly when you have more experience and time in with the company. Many people tend to bring their personal issues to work and cause these types of situations. When the supervisor and president get manipulated and pulled into the situation, there is not much that can be done without your daughter looking bad.
The best option for your daughter is to hold her head up and do the best job she can. She should not comment any more about the incident and continue to work as if it did not occur. Any further mention of the incident by your daughter may make her look worse.
If she starts to notice a pattern of favoritism of this co worker, your daughter can start keeping a log. If it is something that can be proven and goes against company policy, then she may be able to approach human resources and have a meeting with someone there. She can also contact the EEOC to get advice about how to best handle any unfair treatment on her job.
She may also want to consider searching for a new position. Once a person gains favorite status in a company, it usually spells trouble because they continue to use their position to retaliate and gain more favor. Let your daughter know to keep her job search quiet as it could jeopardize her position if it gets out.
Also, suggest your daughter relieve her stress outside of the office. Do not talk to others in the company about the situation but instead talk to friends and family outside of work. Also, she should practice relaxation if she feels it would help and also get involved with activities that help her deal with her stress.
I hope this has helped,