England, inheritance from an artist and inheritance tax

Asking for a family member, he has inherited from an artist, all of said artists art work, including taxidermy, canvases, sketches, oils, watercolours, lithographs, literally hundreds of pieces. This is an unknown artist, but people who he has shown the work to think it is very impressive and could sell for increasing prices in the future.

At the moment the inheritance is valued(by the probate solicitor) at less than the IHT threshold, but if the value increases in the future - if he manages to eventually sell it for a lot higher than the IHT, how does that work for IHT? is there a time limit, if in 10 years he sells pieces for thousands an manages to make half a million, or a million or more out of it, will the tax man want more money?