Actually a lay person can perform a “Minor Exorcism” as seen on the USCCB website:
The minister of a minor exorcism is the designated authorized minister of the sacrament (RCIA or Baptism for Children) or blessing being celebrated. Thus, the prayers in Appendix II, “Supplications which May be Used by the Faithful Privately in Their Struggle against the Powers of Darkness” may be offered by any member of the clergy or by the lay faithful. However, the Rite of Major Exorcism is to be celebrated only by a bishop or a priest who has obtained the special and express permission of the diocesan bishop.
Granted that’s not really what we are talking about but it is technically a exorcism. Maybe that is what the above poster was referring to.
Related Canon Law:
Can. 1172 §1. No one can perform exorcisms legitimately upon the possessed unless he has obtained special and express permission from the local ordinary.
§2. The local ordinary is to give this permission only to a presbyter who has piety, knowledge, prudence, and integrity of life.
This was not actually always so and historically lay persons could perform an exorcism, but to curb abuse and because its generally a good idea the church decreed that a priest should be the one doing it. That being said Laypersons, religious, and deacons often assist in exorcisms. Some Exorcists also have lay prayer teams or discernment teams they send to discern if an Exorcism is needed before they go.
It should be noted this is related to using the actual Rite of Exorcism. Any Christian can struggle against evil including a person under Demonic oppression. So don’t think you are helpless just because you cannot perform the Rite of Exorcism. Some methods that have proven most successful are the St. Michaels Prayer, Rosary, Brown Scapular, & St. Benedict Medal.
That doesn’t mean you should go looking for trouble of course, as seeking out any confrontation with evil is extremely dangerous and should only be undertaken out of necessity in dire circumstances and you should immediately try to get clerical assistance. You can ask anyone who has actually encountered a true demonic situation and they will tell you it is not fun and not something you should seek out on your own.