The Garmin HRM straps have a lot of adjustability, so if she's so small that the strap is too big for her, maybe the electrodes are just too far from her heart to register?

(For reference, I wear a 34-band bra, and my Garmin HRM strap could be shortened at least 4" from where I wear it.)

I know it's tough to give up on the GPS, but maybe a different brand HRM might fit her better. Maybe one of those sports bras that has the pocket for a Polar HRM right in the band.

I don't know about disassembling the transmitter to be able to use EKG electrodes, but if you could find a used one cheap on eBay or whatever, it might be worth giving it a shot...