Mono shouldn't affect the heart. If there's something going on with the heart (irregularity, etc.), that should be checked out separately. Unfortunately, mono won't help with her situation, but it's not the cause either. Hope that helps!
I have a 19 yr old daughter in college and I have to say am so thankful we finally have her medical disability in place so she rarely ever goes to class. It may not be much of a life but until they find something to help her it works her staying in bed. So far the say she has Narcolepcy and hypo somnia. But that doesn't explain her being ill for 6 years...! I pray they find help for her! As for your lovely daughter she should ask for a holter for a 24 hour heart test. I read earlier this week that is the one test everyone with CFS fails...although I also found out that you don't want that in your records about the CFS... It follows you forever... and hard to keep a job or insurance. Good luck