First of all, there is no such thing as a Christian medium. The bible explicitly condemns mediumship (see Deuteronomy 18:10). Summoning the dead, which is known as necromancy, is extremely dangerous because it calls upon demons who masquerade as the dead.
As this blog explains, the deceased have no power to appear or communicate to anyone after they die. In order to do so, they must rely on either a supernatural (God) or preternatural (angels/demons) power to do so. Because God explicitly condemns this practice, we can be sure He will not facilitate anyone’s appearance to a medium, nor will He allow His angels to do so. This leaves only Satan who has the power to facilitate such an appearance. For this reason, the Church has always warned the faithful to stay away from necromancy, which includes the work of mediums/psychics and channelers.
As for this so-called prayer of protection, this is also a misnomer. Only God and His angels can protect someone from the kind of harm that could be done to them by a preternatural being. Relying on some kind of "white light" for protection is not only nonsensical, it is also very dangerous and in direct violation of the First Commandment.
But tell your coworker not to be too hard on his mom. Many Christians are poorly catechized about the faith, which means they also have no clue about the occult, Satan, and the nature of evil other than what they see on TV (which is mostly written by people who don't have a clue either).
This explains why there is such an explosion of spiritual maladies in our day and time. People think they can just “dabble” around in occult-based practices such as Ouija boards and tarot cards and other forms of divination with impunity. Nothing could be further from the truth. Whenever we “dabble” in the occult, we open a door (known as a portal) which allows the devil to have greater access to our lives. This door remains open until it is closed. The only way to do that is to reclaim our baptismal promises and renounce these practices. We opened the door with our own free will, and so we must close the door with our own free will.
For this reason, anyone who has “dabbled” in the occult – participating in anything from palm-reading to horoscopes to séances and psychic readings – needs to 1) personally renounce the practice and ask forgiveness from Jesus Christ, and; 2) confess this sin to a priest.
As for resources, My Learn to Discern Compendium explains the occult in very down-to-earth language and offers in depth reports on several familiar practices such as necromancy, ouija boards, psychics/mediums, tarot, etc.
John Labriola's book entitled Onward Catholic Soldier, which also explains the occult in every day language, is a personal favorite.
But after you read these two books, you'll want Paul Thigpen's Manual for Spiritual Warfare!