Settlement vs Subsidence

What do you mean by subsidence and settlement?
What's the difference?
You need to ask the appropriate dept. of youir local council.
It's not a question for an amateur.
There are many reasons for both.
Just look at the tragedy in Florida. Huge tower blocks built on virtually sand.

Depends where the cracks are, are they travelling diagonally along a wall from the ground up? Are they visible from the outside? Is there a problem with doors and windows? If he doesn't actually own the property it's not really too much of a problem, or if it is a problem then the landlord needs to get a surveyor in or contact his insurance company. If he has one of course.

Small cracks are probably settlement due to weather conditions and can affect houses of all ages. If they are very deep then subsidence could be the cause. A surveyor could tell for certain.

We used to get cracks in the top of the walls in an old house. It turned out that the upstairs floor wasn’t tied into the walls and the wall was bowing out. We had wall ties fitted and all was well.
Unless the landlord is a professional surveyor I shouldn’t think he’s in a position to comment. There are all sorts of reasons this could be happening - it needs checking out by a professional surveyor.

Subsidence would be large cracks affecting the structure of the house action would be needed for safety
Smaller cracks that can be covered up by decoration are not a concern unless the reappear larger.

Thanks all, I'll let him know.

Houses do settle over time and it can continue even after 80 years if there are major changes in local weather or there has been construction or other work in the area that might have disturbed the water table, or if it is in an area where there has been gradual land slippage.

Recent extremes of weather, prolonged periods of heavy rain or droughts can lead ground to swell slightly or dry out. It will depend on the underlying geology

Subsidence is the same but different. Sobsidence usually happens in parts of the country where the geology is a mix of clay and gravel and lens of clay lie within the gravel. It is particularly prevalent in the Thames Valley, here when there are prolonged periods of rain, the clay lens swell with water and that leads the gravel to deform and it can be severe enough to cause problems with house foundations. Alternatively in years of drought, the clay shrinks and the gravel collapses into the space left behind

Other things that can cause subsience are where houses are built on unstable ground, or built over patches of different geological materials. Leaking drains can also excavate holes underground. This happened to a neighbour, a waste pipe had broken underground and all the water had washed much of the soil under their foundations away.

If your neighbour is worried the cracks are growing, the best thing to do is to get the landlord or a surveyor to attach some glass tails across the gap. If the these break then there is a problem with current movement if they do not, all is well.

Of course it might just be because the interior of the house has become more dry. Better heating, turning the temperature up permananetly or having the boiler on 24/7 when previously it has been only on twice a day, would dry the atmosphere and draw moisture out of the walls. We live in a very old house and were the first to install central heating in it, we had cracks in the plaster everywhere after we had that in place for a season, as the house dried out now it was kept at an even warm temperature. Adding insulation or draft exclusion could also have that effect.