My friend is kind of messed up in the head from some type of mental illness, but he’s a nice guy.
精神 is your spirit, both religiously and spiritually. When those are sick, you have a problem. Back in the day, a mental illness was thought to be a sickness with your spirit. Nowadays you can just call it a mental illness or mental disorder.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Will help if you know 精神 already, though.