Joe, ya got it all wrong. IMHO.
The OP has already proven to the neighbor (in the neighbor's mind) that she/her family can't take care of herself, because she admits that she needs the neighbor to plow her driveway.
Now, this is a country patriarch.
So --
In his mind, he already knows that she/her family can't take care of herself. To him, therefore, it logically follows that she/her family also can't take care of her field -- so he mows it. And since, in his mind, she now can't take care of her driveway OR her field, it also logically follows that she can't take care of her trees either.
He may be patronizing and overbearing, but in his mind he's just being a good friend to her.
IMHO, being aggressive with the neighbor wouldn't make anything better. I'm with the folks who suggest pie and/or eggs, plus effusive thanks for plowing the driveway -- FOLLOWED BY an honest but NOT confrontational discussion of the field and trees. They need to work TOGETHER to keep a good relationship going, NOT fight against each other and possibly create a chronic feud.