Hmm, a bit of a problem. Your family is going to have to agree to settle this amicably.
It is the estate of the deceased that pays the IHT - the notion of 'each party' (being the beneficiaries) doesn't come into it. Thus unfortunately it seems the will has been drawn up without thinking through the implications of increasing house values. If one beneficiary has been left 'the house' in the will, then that is what they get - after IHT has been paid out of the estate. Which is going to mean the residue of the estate is pretty small by comparison.
It may be possible for a deed of variation on the will, if all parties agree and the family / beneficiaries all agree they want to equalise the wishes of the deceased.
PS - has the friend appreciated that there may be transfer possible of part of the nil rate band from the first-to-die (the father), such that the £325k allowance you quote may be higher than £325k before IHT is liable on the residue??