They should have asked very nicely her to move, and offer her a glass of bubbly or something for the inconvenience. They are stupid if they expected an empty public park. They wanted a cheap day, but expected to bully others because of their number, I hate that attitude.
It's the same thing with people marrying on the beach. In an ideal world, they would be deserted, in real life there are tourist lounging in the background.
It remind me of parents who refuse to pay to reserve specific seats on a plane, because they expect others - who have paid- to move and let them seat together.
All these people think they are clever because they are getting something for free out of the kindness of strangers. Good to the sunbather not to have been intimidated.