The cross trainer will give you a better work out as it will also involve the upper body as well. The minimum exercise level for just normal good health is 30 minutes 5 times a week, so basically you need to exceed that to get anywhere.
I am afraid this does occur in a small number of women.It might be worth her trying a progesterone only pill,but if this causes the same issues a non hormonal method of contraception is the only option.