The witness proved that Viet was guilty.
In my husband's court case, the clerks of the bank where he has his accounts appeared in court as witnesses for him.
As a witness, I won’t beat around the bush, I’ll answer your question frankly. Yeah, that Fugu was absolutely eating your pudding.
An evidence person is a witness, because witnesses are people who provide evidence (you know, with their mouths or something).
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.