Shalom and thank you for your question!
Buying a Mezuzah as a house-warming gift is a very worthwhile idea. Psalm 121:8 states “The L-rd shall guard your going out and coming in from now and unto eternity…” Affixing a Mezuzah on the doorway is one of the ways to draw holiness into the Jewish household. It is also a hallmark of Jewish identity.
There are various laws and customs concerning the commandment of affixing a Mezuzah. The front doorway is not the only doorway that needs a Mezuzah according to Jewish law. All the doorways except for doorways to bathrooms or changing rooms require Mezuzot.
A Mezuzah needs to be procured from a reliable source. Let’s start from the beginning. What IS a Mezuzah? Some people may not realize that an attractive case may be hiding nothing at all, (in other words the scroll of parchment which is the real part of the Mezuzah is not inside, G-d forbid,) or a piece of printed paper, instead of a scroll of genuine parchment written on by the hand of a genuine, qualified Jewish scribe, called a ‘Sofer’ in Hebrew. Unfortunately, the market is flooded with forged Mezuzot, some of them printed, others written by hand by unqualified people. The scribe, or ‘Sofer’, needs to be a G-d-fearing individual who is very careful to observe the commandments of the Torah in general, and the specific laws of writing a Mezuzah in particular. The parchment should preferably not have been treated with a substance that makes it easier to write on, because this substance can shorten the life of the Mezuzah, by causing more sensitivity to climatic changes or humidity, so that such Mezuzot may last only one year. Once affixed, a Mezuzah needs to be checked at least twice every seven years, but preferably once a year before Rosh haShana, the Jewish New Year. This is because the above-mentioned climatic changes or other factors may cause problems even in a Mezuzah that was originally kosher.
The local Chabad house should be able to refer you to a trustworthy place where you can acquire a kosher Mezuzah. They or someone they may refer you to, can explain how the Mezuzah should be affixed, the blessing for the home-owner to make, and so on. Another important factor is whether to affix it on the right doorpost or the left, and the law is on the right of where the door opens in to. The front door, therefore, usually has the Mezuzah on the right hand side of the entrance from outside. The inner rooms may differ, according to the direction that the door opens to – to the inside of the room, or to the outside of it.
Having emphasized the importance of making sure it is done in a kosher way, we wish your friend a good and happy acclimatization to his/her/their new home, in good health and happiness, and the same for you!
