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However, when I saw the disappointment in her eyes/face, I knew that accepting her gift was the best way to thank her.

Are the underlined versions very idiomatic? If not, how would you revise them? Thanks.
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I don't think it's an idiom at all, but literal.

I'm not sure how her being disappointed can be fixed by accepting her gift or what that has to do with thanking her, but the part about being able to see the disapoinment in her eyes is perfectly natural!
Comments  
I think that these are idioms, but idioms which are easy to understand, so not "very idiomatic".

"When I saw how disappointed she seemed to feel" or "When I saw her disappointed expression" would not be idioms.
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 BarbaraPA's reply was promoted to an answer.
Grammar GeekI don't think it's an idiom at all, but literal.

I'm not sure how her being disappointed can be fixed by accepting her gift or what that has to do with thanking her, but the part about being able to see the disapoinment in her eyes is perfectly natural!

Thanks, GG.

Got it.
My eyes are never disappointed, and the elixir of disappointment never enters them. They are just squidgy, unfeeling balls of jelly.
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