his/her opinion
Según su opinión, él es el mejor músico que ella haya visto.
In her opinion, he is the best musician she has ever seen.
They should express their opinion.
Tom le pidió a Mary su opinión.
Tom asked Mary for her opinion.
Si cambiara su opinión, hágamelo saber.
If you change your mind, please let me know.
I'm not asking for their opinion.
I was just repeating his opinion.
Tom se guardó su opinión para sí mismo.
Tom kept his opinion to himself.
His opinion was not accepted.
This is their opinion.