That is probably going to be the key.
I am not really leaving anything out which I think is relevant.
I am just sussing out the risk picture.
One problem I can see is that if upon death the estate is worth say 50 grand below the IHT threshold, then anybody wanting to sh*t stir could tell HMRC about the 100 grand loan.
But if the loan is more than 6 years old on death, which it probably will be, and the old girl never asked for any repayments, it cannot be due anymore, so it cannot be a part of the estate.