说 Sb is honest to do sth与It is honest of sb to do sth都可以的。你在词典上没找到可能是你的词典没有收录。

Oxford Practice Grammar (by John Eastwood) 一书有如下说明（其归纳的形容词中提及了 honest），可以证明这个句式的正确性：

After an adjective saying how someone behaves, we use of + object + to-infinitive.

It’s kind of Melanie to put you up for the night. (Melanie is kind.)

It was clever of you to work out the answer. (You were clever.)

Some of the adjectives we can use with of: brave, careless, clever, foolish, generous, good, helpful, honest, intelligent, kind, mean, nice, polite, sensible, silly, stupid, wrong

关于 Sb is honest to do sth 的用法，《简明英语惯用法》（葛传椝等）上面有这样一句：

He was honest to tell us the truth.