She wants things done but can't be bothered with the paperwork or the general faff of doing it herself? Then she can appoint anyone her agent, simply by writing and signing a document saying that they are that for the purpose in hand, but a power of attorney is generally required if the transaction involves land.
. If the present POA is dependent on her being no longer of mental capacity before it becomes operative, then she'll have to make a specific power of attorney while she has capacity now, empowering the attorney (the person named) to act, in her stead, in whatever way she authorises.
Reducing liability to inheritance tax is a big area of study. Simplest is giving the estate away, thereby reducing what's left to below the limit for IHT, whilst ensuring the testator retains no interest in, or benefit from, what's given, and then for the testator to live 7 years. Other ways are buy or have a farm which the testator actively farms for a sufficient period to death (that is directs; being an absentee who leaves everything to contractors won't do) and have a trading business company which the testator controls and passes on in the will. Farms and their farmhouses and trading businesses attract 100 per cent relief against IHT