Recommended shared calendar for iPod, Android and Windows?

Hello All,

A friend’s father has doctor appointments, home health care, prescriptions to be picked up and myriad other items to keep track of.

His adult children like to help out driving to appointments and simply making sure Dad has somebody around fairly often.

Can you recommend a good shared calendar so everybody can easily see what’s on Dad’s agenda?

I picture this:
—Everybody can see the upcoming schedule from a smartphone or web page
—Everybody can add events and edit events
—It is REALLY easy and intuitive to use, if it’s even a teeny bit complicated people won’t use it

Any recommendations of tools you like (or don’t like) for this?