[sc_rufctr]

The way I explain this to myself that makes the most sense.

1. The people that stole the coffin are in the funeral trade and they know they could resell the coffin. (Most likely over and over again.) They also have the lifting/digging equipment to get the coffin out of the grave. All they'd have to do is surveil the family and steal the coffin when the family goes on holiday or they're out of town. A solid granite coffin would be easy to clean, polish and refurbish to look like new.

2. The mother may have dealt with someone who was on the team of coffin thieves at the time of her daughters death. And for whatever reason she may have sensed something was off but dismissed it or couldn't recognise it because she was grieving. As she recovered from her daughters death the dreams start because subconsciously she knows there was something off about that person she spoke to.

We all know grave robbing has been going on for thousands of years. Why wouldn't it still be happening now? Especially considering that in this case there was a very expensive coffin just sitting there under the ground. I've seen people do some horrible things for a few dollars. What would they do for $100,000?

But if you ask the mother she'll tell you her daughter was talking to her from the grave.
... And good luck trying to convince her that there may be a reasonable explanation.