Your friend has just as much right to live in the house as his wife does. In fact, he has a right to break in (I'm not necessarily recommending that,) and if she changes the locks he can call a locksmith. For the same reason, your friend cannot have his wife removed. The only exceptions to this are if there is an Order of Protection in effect, or an Order for exclusive possession through the Family Court. It sounds like someone needs to file for divorce.
If the parties are married and the home was purchased during the marriage then both parties have the right to reside in the home. However, if the parties are not married then the person who owns the home has gthe right to reside there. Your question did not have a lot of facts or information. If parties are trying to get each other out there may be more problems here. I suspect possible domestic violence or threat thereof. If thisis the case then it is possible one party could be removed by order of court with an Order of Protection which can be obtained at the local county courthouse. I suggest yoou tell your friend she needs to consult with a lawyer that has expertise in family law ASAP. You also mentioned nothing about children. The court also weighs the hardships to the party being removed and whether that party hs anywhere else to go and stay. She needs to consult a lawyer now before things get worse here and someone gets hurt. You can go with her to the lawyer for moral support. It is great that you are helping your friend. She needs it! Good luck!
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Since they are married, they have equal rights to live there unless there is a court order to the contrary. It doesn't matter whose name is on the title or who is on the mortgage. It sounds like there are some serious issues going on. Your friend should talk to a lawyer, providing all of the details for specific recommendations as to what should be done.