Subsidence is caused more by underlying geology than how much water you put in the ground.
We used to live on the heathlands of the Surrey/Berkshire/Hampshire borders and subsidence was a problem there because the subsoil was a mix of layers and lenses of sand and clay, and the clay shrank and expanded when it was wet, while the sand didnt. I lived there more than 20 years and I never heard the idea of watering the ground put forward as a way of stopping the shrinkage and expansion.
We then moved to an area where the geology was uniformly clay, again, we were there more than 20 years. We lived in a road of medieval houses and none of them showed any signs of settlement, and they were up to five hundred years old.
Personally I think your friend has been told something she misunderstood. If houses in the area you live in have never shown signs of settlement in the past, what she is doing will be good for her lawn but have no effect on the stability of her house. If the house is in an area that does have settlement problems, then the amount of water she can put in the ground, will be only a drop in a small reservoir of water that would be needed to solve what is a geological problem over a large area.
All houses, old and new, settle a bit in their first 5 years, but watering the lawn will not alter that.