Are you referring to lattice? If so, yes, it needs to be treated. You can buy pressure-treated lattice at any of the Home Centers.
It comes in 4' x 8' 'sheets'. If you need to cut it, you cut it to fit. If anything, it's gonna shrink. A circular saw, or any power saw is much better than a hand-held one. YOu can get away with a hand-held saw but your edges won't be as straight. And depending on what you're attaching the lattice to, I'd use short, plastic-coated deck screws. THe screw needs to be as thick as the lattice, PLUS 3/4".
My best to ya and hope this helps.
PS: If you're really talking about trellises, you'll need to prime the wood and apply at least 2 coats of top-coat. Over time, the trellises may need to be repainted. Are you sure she wants to 'remove' or 'kill' her plants to do this?! Otherwise, in time, they'll need to be REPLACED! Consider vinyl-made trellises ...