I want to tell my friend that he could do better than his fiance
Why would you tell your friend that he can be with a better person than his fiance?
He needs to hear it because his fiance isn't that nice to him
That is his decision to make. If he wants to marry someone, it is his choice and not your business.
I will tell him anything I want because it's my opinion, and it's only an opinion
It is an opinion that could hurt his feelings. It can seem like a betrayal of trust if you say that to him. Support the decision he makes.