hi John,
Some of our friends in here have already quoted you prices based on their experiences and capabilities. I think it will be better if you ask your wife's friend first how much she/ they can afford. I am sure ( most of the people in here will agree with me) bringing up a child is really expensive,and as much as possible you want what's best for your child.
There is really no specific amount for a child's need so, if she can afford to give let say £50 or even up to £150 monthly it is her decision,just make sure that the money is really going for the child's needs not for any other reasons though I am not sure if she/they discussed it with her family even before she leaves for Spain?..