Only thing I’ve ever seen that doesn’t require a chest strap, but works well during exercise (ie. If you are moving your body around) are 3 wire electrode ecg units. They are a lot more hassle than a strap – you have to stick electrodes over your chest. They are also very expensive (we have a cheap one, costs 600 quid, and a medical grade one, costs 15 grand).
There are touch ones (which suck) , finger clip ones and ones in headbands using infrared blood flow monitoring, they are also a bit useless during exercise.
She should try a chest one before deciding they will be useless – we’ve fitted various forms of hr monitoring to loads of variable sized people, including fat male celebrities and page 3 girls (some pr stunt), both of whom I’m pretty sure had larger chestal areas than the average, with no problems.