You can cut back plants or trees overhanging your property but you must give the bits you cut off back to the owner. I believe this is the law.

If she has cut the plants back beyond her boundary line there could be a problem. I'd apologise and see if it can't be sorted amicably.

One interesting point is to find out who the wall belongs to - if it's your mother in law's, he hasn't got a leg to stand on - in fact, he should not have attached a trellis to it at all, but to a separate support on his side. If the wall is his, then what Moongrower says is accurate - you can cut back plants that overhang your garden within your own boundary line (if you imagine it rising vertically from the fence, wall, or whatever) so if she's gone a few inches over, he could make a fuss - but by the time he chooses to take legal action, its entirely likely that whatever she cut back will have regrown anyway, hopefully.

and really not worth the hassle... how can they really know she cut back too far!