Is an individual retirement account considered shared property in a divorce?

My friend is getting a divorce; She's got her own retirement account that's been accumulating over about 20 years.

She and her husband had a house which she is not interested in keeping, and he is, so he is asking for all her financial records, including how much she has in retirement accounts, in order to "make her an educated offer" on buying her out of the mortgage that they're on 50/50.

I'm trying to figure out why the assets of the person being bought out, would have anything to do with calculating an offer based on the home / mortgage / equity value.

Can anyone shed some light on the legality of this? They're trying to do it without lawyers.

Thanks!
Added (1). In CA btw:)

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