If the holiday home is gifted to the person 7 years before death, there is no IHT to pay.
If someone dies, and they have not left everything to their spouse (they may not have one) then IHT is payable on the whole estate above the threshold. If it was all left to a spouse, no IHT is payable on the estate of the first death.
You have to understand that it is the estate that pays the tax. If there are not enough liquid assets to pay it, the executor has to sell something to raise the money. Where else would the tax come from? It would have to be a fair arrangement for all the beneficiaries, not just the person getting the residence or the holiday Home.
Therefore anyone making a will really needs to understand who will actually get what out of their estate. If, for example, the holiday Home was given 7 years before death, as a gift, it is exempt from the estate and IHT. Just the main residence and the assets would be liable to tax. But is this fair? The holiday home owner gets it but the main residence is sold to pay the tax. The tax liability would be less but only some beneficiaries lose out by having an asset sold that was left to them.
To minimise IHT, there needs to be professional help with IHT tax planning and the will for this estate. All the assets and residences should be considered and if the person making the will wants to ring fence the holiday home they can give it away 7 years before death. If they give their main residence away but continue to live there, they must pay a market rental to the owners.
The main thing is, IHT is payable on an estate, by the estate, not by the people who inherit. The executor administers it.