A friend went to psychic years ago. I lost touch with her. A I responsible for trying to contact her to tell her it's a mortal sin?
You say you’ve lost contact with your friend. Do you know how to get back in contact with your friend or know someone who is likely to help you in succeeding in that request?
If you can answer one or both of those questions affirmatively, then, out of charity, it’d be good for you to let your friend know to avoid psychics and trust in God, instead of consulting problematic spirits to deal with life issues (see CCC 2115-17). Yet, make sure to approach your friend with humble charity, including because it’s been a good while since you’ve last spoken (see 1 Pet. 3:15). If your friend is Catholic, encourage her to take advantage of the sacrament of reconciliation. If she’s not, encourage her to repent to God on her own.
Further, consulting a psychic is certainly, objectively, gravely wrong. However, remember that mortal sin involves two other components: full knowledge and full consent of the will (see CCC 1857-61). Consequently, while encouraging your friend to repent and avoid this sin, don’t presume to judge her subjective culpability.
If you are not able to contact your friend, pray for her repentance and conversion.