Thanks for your query at DoctorSpring.com
With irregular cycles, it is just not possible to guess the days of ovulation.
Did she always have irregular cycles?
Irregular bleeding is not normal for her in the late 30s.
It warrants investigations.
Some common reasons for irregular cycles are = stress, anxiety, emotional turbulence, weight changes, unaccustomed exercise, travelling, climate changes, changes in sleep or diet, hormonal medications etc.
Other serious reasons could be - polycystic ovaries, fibroids, cervical lesions, endometrial hyperplasia etc.
She should be consulting an OB - GYN and having the following workup -
Pelvic ultrasound scan
Hormonal levels - FSH, LH, TSH, Prolactin, OGTT, DHEAS, Free Testosterone etc.
For confirming ovulation, she should be undergoing follicular monitoring, which is the only reliable tool for confirming ovulation in women with irregular cycles.
Follicular monitoring involves serial ultrasound scanning of the uterus and ovaries, to check the growth of the egg follicle.
All the best
Please feel free to discuss further.