A question would be whether the ex wife is getting the $43K because a) she's entitled to 1/2 of the marital portion of his retirement account or b) they're using it as available funds to finance a division of property (for example, he's keeping the house so she's getting money).
If the intention is a), then, in my opinion,it's 100% correct that she should be the one who has to pay the taxes (now or in the future) on her portion of the retirement account. She wants 1/2 of a tax-deferred asset without owing the deferred tax.
If the intention is b), then the question is whether they allowed for the income tax effect in determining the $43K in the first place. For example, if she was supposed to get $35K but they added $8K for taxes, it would be double dipping if she makes him pay the tax now.