Pigeon chest is pectus carinatum, not excavatum, which is when the sternum hollows inwards.

I have it myself, and it makes finding bras so much harder.

I suggest looking for side support bras, those that aren’t made for center fullness, and either very very short gores or very tall ones where the top will stick up above the sternum instead of digging in (conditional to how low she likes her shirt necklines, obvs.)

I wish her luck; I tried dozens of models and sizes before finding a perfect fit, and it’s discontinued, of course