Heart arrhythmia question. how can i help my friend if she doesn't take her meds?

Can't: "you can lead a horse to water but you can't make him (her) drink." doctors face this problem every single day! if the rhythm problem is not of the life threatening type, your friend may simply find the symptoms less onerous than the medicines. So be it.

You probably can't: Unfortunately, many times we don't learn by other people's mistakes and no matter how much you try to convince someone of something or how much evidence you present, some people are just not ready to accept reality and their way to cope is to deny. Until your friend accepts that she has a problem (that can fortunately be treated or ameliorated with meds) she won't comply with the treatment.