Has the plant ever been treated as a type III clematis and pruned back hard during its first few years in the ground? This helps the root system get big enough to support all the top growth. If the roots can't support the top growth, then some of the top growth wilts out so that the rest of the plant can survive.

In my area, when we go from coolish weather to warm weather in short order and the humidity gets low especially when it is windy, plants will often wilt back due to the extreme condition changes.

If you know for a fact that the plant does not need water, then perhaps there is something chewing on the roots or even on the stems down low on the plant. Check for breaks or creases in the stems that might be caused by wind damage.

My last piece of advice will probably not be to your liking, but the type IIs, in which Nelly Moser falls, often have issues that the type III clematis do not. Consider suggesting to your daughter that she grow some of these types of clematis and see if that helps with the issue.

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