I can relate to your performance anxiety myself. I think it is more about the feeling of being judged and the thoughts of those in the audience then it is about your capability.
I would not try anyones medicine with or without a performance. Performance is more about your self esteem and belief in your talents then any medicine you can put in your body.
Capability and Self Esteem are also 2 different animals when dealing with anxiety. For instance, you know you are capable of playing many types of music, but low self esteem puts you at risk for not reaching your full potential because of how you perceive the rest of the worlds thinking about you.
I read that your performance is coming up soon, but it is never too late to look into meditation to get yourself into a more comfortable zone where the audience is just listening to you and not judging you. I have also found that playing extensively before the actual recital relaxes and also invigorates a person as they feel more fluid then just going out on stage cold ...
Good luck to you
Don't take your friend meds if it was not prescribed to you. Go see your doctor and ask about the beta blocker which is true can really help. A resting heart rate of 50 is fine if you are athletic and your heart has a stong stoke volume. Some serious atheletes run around 40 at rest.
Whats your blood pressure when your heart is at 50 and you are at rest?
Clonazapam is a long acting benzodiazepine that may help, If you ask the doctor that you only need something during performances and it is not an every day thing then maybe that could be an option.
My blood pressure is 116/61 at rest. My pulse is actually fairly high now at 50 as i haven't exercised much in 6 months,it used to be between 40-42, i've started running and cycling again so am expecting a fall to low 40's in the next month or two.