Hold on, hold on, let's see if we're on the same page here.
She hires Joe, let's say. She pays Joe, and Joe cuts her grass.
The park says she owes money because they have a guy, Steve, to cut her grass. ...Except Steve says he's never cut her grass before, since it's always nice. (Because Joe's already cut it.)
Check the lease, because something's fishy here.
It could very well be legit for the park to say "If you don't keep your lawn nice, our guy Steve will cut it, and then you'll be charged $100 for his services." That might be considered a fine, or even a bill for services, them charging her for Steve's work because she didn't take care of it herself. But it would be covered in her lease's terms.
The lease might also stipulate that she's to pay some amount of money into a service, whether or not she takes advantage of it, like Steve's lawn care offerings. IANAL, and I'm not sure how legal that could even be. But the instances of this that I've heard of have generally had it be per month, not per mowing, and they're claiming they came to their $400 total because it was four mowings, correct?
So saying she's being charged a surprise $100/mowing, for four mows that never happened? That's not legit. Especially when Steve agrees that he never did the mowing. That's called trying to charge for services that were never provided.
TLDR: the lease matters, but it sounds from what little I know like they're trying to charge for their guy doing work... that he never did.