If she’s already badmouthing you, think how much worse it will get after the Wedding, when she really has nothing to lose. Even if you let them have the field in perfect condition, you will be accused of deliberately doing something to spoil it. The grass in the field will be too long, the ground will be too soft or something else.
If your dh foolishly insists on giving them use of the field, you definitely need to get them to sign a watertight contract, ensuring that the field will be returned to its previous state, or they will be liable for the costs of doing so. Of course a large bond, of several thousand will have to be paid into your account, from which any damage or clean up costs will be taken, before the balance is returned to them. Make sure there are plenty of photographs, so there is proof of the condition the field, gates, fences etc. we’re in, prior to their setting up.
Also insist that they take out liability insurance, which will cover any injuries or accidents that may happen on your property, a copy of which must be provided to you prior to their setting up. Obviously no electricity or water, or entry to any other parts of your property, including your home, will be made available to them.
They may hopefully decide not to use your field, when they know that they will have to clean up all the mess made after their Wedding, or they will have to pay the costs of any outside contractors, that are called in to return the field to the condition it was in prior to their Wedding.