Writing thesis that rebuts advisor's theory
There is no predicting personality without close study, but if the person is a true scholar they will welcome an advance, even if it refutes something they did earlier.
No, hostility isn't an unreasonable possibility. But whether it is likely depends on the person. Impossible to say how likely. You are in a better position to judge, knowing them. Are they insecure? Do they welcome a challenge? Are they fair and honest?
But it would probably be better to approach them with a questioning attitude rather than one making bold claims. After all, you might be wrong, as you say. And you should be sure of the wider literature. Has the theory been validated by others? Accepted as fact?