That's a very good point.
5 years ago, your friend's mother was worth €600,000 - the price of her house.
Today her house is worth €300,000 and she has a loan of €140,000. So her net assets now are only €160,000 - assuming her mother has spent the money.
However, I think, it is in your friend's interest for house prices to continue to fall, as it will be cheaper for her to buy the house. Against that, her share of the estate will be less.
Your friend should be concentrating on her mother's welfare and if her mother spends all her assets before she dies, then tough on your friend.