[susan53]

Yes. by can be used because they are both essentially passives - eg

Her attitude surprised him
= He was surprised by her attitude.

The suggestion shocked her.
=She was shocked by the suggestion.

But at is often used to mean because of - eg She went at his invitation. / He laughed at the idea.
- ie the invitation was the cause of her going and the idea was the reason why he laughed.

The same seems true here - her attitude was the cause of his surprise, and the suggestion was the reason she was shocked, which gives us :

He was surprised at her attitude.
She was shocked at the suggestion.

So either can be used. The same is true incidentally of various other past participles which are used adjectivally - eg appalled /delighted.

She was appalled at/by what he said
She was delighted by/at what he said.

But it doesn't always work - it's not possible with eg scared or interested which can only be followed by by and not at.

He was scared by the storm.
She was interested by what he said.

And some participles can take other prepositions too - eg : delighted with, interested in

She was delighted with what he said.
She was interested in what he said.