All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
According to the view of the majority of the jurists, it is not a condition for the validity of divorce to have witnesses for it, because divorce is the right of the husband [i.e. it is in his hand], so whenever he issues it, it is effective (valid). However, having witnesses for it is desirable.
Allah Says (what means): {And bring to witness two just men from among you.} [Quran 65:2]
Nonetheless, divorce is effective even if the husband did not bring two just witnesses.
However, it is better to have witnesses for it in order to avoid the difference of opinion.
For more benefit, please refer to Fataawa 133297, 88655, 91187, and 89894.
Allah knows best.