As you have mentioned the tests she has till date undergone are for her cracked heels and are normal.
As her age is 60, most of the times after menopause
there are thyroid
problems. Please do get her TFT(Thyroid Function test) done. Her fatigue
and weight loss
might be due to that.
Regarding the cracked heels, the simplest way along with vitamin B12
is soak her feet in warm water for 10 minutes and then scrub them every night before sleeping. After that dry them apply petroleum jelly and wear socks. This might sooth her heels and remove the soreness.
Hope this answer helps you.