One of the key things to find out is if the house is owned by the couple as joint tenants or tenants in common. This is A legal status and is more important than who is paying what.
Some comments have touched on this but some clarification is needed.
If joint tenants, on the first death it will automatically pass to the surviving spouse. This cannot be contested. The survivor can then leave it in their will to whoever they choose. Or if they die intestate, it will follow the laws of intestacy,the most likely scenario being that the survivor’s children will inherit, if the survivor has not remarried.
If the house is owned as tenants in common, the couple can state in what proportions it is owned, commonly but not necessarily 50:50. It is normal for a will to make provision for the survivor to have the right to remain in the property, pay for its maintenance and insure it, until death or remarriage. On the second death, the shares are inherited according to each spouse’s will.
It is perfectly possible to change the way a house is owned and once it is owned as tenants in common there will be a marker in the Land Registry to prevent the property being sold without the consent of all relevant parties.
For example, As we own our house as tenants in common, if I die first, my husband still owns his half and can stay in the property, but when he dies, it will be sold and the proceeds inherited according to our wills. As we each have two children, we have decided that each will inherit one quarter.
I believe it is possible to sever a joint tenancy without the consent of the spouse.
This issue alone needs clarification and the right paperwork. It is a legal matter.
Then there is the issue of the mirror wills and writing a new will. Again, a solicitor should be used, one who is a specialist in wills.
Or more likely the friend should be taking steps to get out of this marriage.
Legal expertise is needed, whatever she decides.