The person with more free time still gets to decide how much of it is available.

Spare hours are not public property. Someone who has retired, works part time or has no children is often treated as the obvious person to take the airport run, the committee or the long visit, as though an empty calendar were an invitation. Time that looks free to you is still someone’s life, and they are entitled to spend it on things you cannot see.

Ask rather than assume, and make no an easy answer. If you are the one with the open diary, practise declining without producing a reason. Not being busy is not the same as being available, and you are allowed to say so kindly.