Being ambitious doesn't mean someone is evil. I'd say it makes them motivated to achieve their goals. The means they use would depend on their moral compass. Which he doesn't know at all. And yet, he's just going to believe everything she says, with no evidence? Like, "oh, this person who can perfectly gauge others' emotions and the way she is perceived, to the point that she is confident in earning anyone's trust in a short conversation, just said something that sounds good to me after she tricked me into telling her what things I value, so I'm going to reveal everything about me in return." And what annoys me is, he likely won't be bit in the ass by this, but he should.
Let's be clear, this is an engagement that was made before they were born, so obviously their parents are very close, which means they are essentially allied and their territories likely share common interests. It would make sense that it's in both their interests to get married. Which actually makes it very weird that no one knew about the engagement, I mean, they're in the middle of a bunch of wars and he has a bunch of genius strategists and politicians already. The existence of an engagement, meaning an allied territory, should have come up countless times already in all these battles they've been involved in for the past few years.
The chance that she will have any reason to betray him is close to zero. She will, however, wrap him around her finger so tightly that he will just do what she wants him to. I mean, he is shown to be an idiot, and he's already bending to her will within the day, even though he started wary of her. And I'm not saying he should keep his guard up forever, but the thing he should do is learn about her, ideally not straight from her mouth. Learn what her territory is like, what her family situation is like, what her upbringing was like, and so on. Basically, get to know her before deciding that the perfect politician likely didn't lie.