Not having direct access to her or any tests available, my money (if she’s a young, healthy individual) would be on Wolf-Parkinson-White. I know you’ve said she’s been to the doc. Did she happen to have an EKG done?
Like I said, this is pure speculation based on what I’ve read. Please don’t quote me on anything here. Just trying to help.
Thanks for the input guys. Checked it again last night and it was 159 at rest. Still not checked her BP though. Yes she is very fit -and looks it too 
and regularly does cardiovascular exercise. I’ll check her HR during exercise when I can. Yes she has had an ECG, but got the all clear. I’ll look up those conditions you mentioned Kelly - Thanks!
first of all Gareth Brierley tell your g/f to see another doc cause i’d be worried and and dude get a sponsor and go into running or some long distant events cause 40bpm in pretty good
I’m at 61. Not bad. Considere that I never have done any Cardio in the last 2 years of training. I started to do cardio i weeek ago with my fat fast diet.
Byp0
WHen I was near-pro triathlete, if given 5 minutes to relax I could get my heart rate into the low 30s. I went into Virginia Mason Hostpital to pretest for a research project, and the nurse that took my vitals said it was the lowest she’s ever seen. Unfortunately, I didn’t get into the project. Not having done much in the way of endurance activity for the last couple years, I shudder to think what my RHR is now.
I got 81 first thing in the morning.
The RHR in the morning after restful sleep can be deceiving. I remember measuring 38 BPM one time, and after I woke up fully, it was 60.