How to calculate ovulation day, because my girlfriend has a different menstrual cycle every month. it goes like this 32 days, 27 days, 31 days, 28 days, 34 days?

Ovulation occurs 14 days before the onset of the next menstrual period. As cited in your example ovulation days would have been Day 18 (32), 13(27) ,17(31),14(28),20(34). Please use contraception at all times if you are avoiding unintended pregnancy as ovulation as you can see is unpredictable. By the way, this daily variation in the menstrual cycle is normal.