<human>: Summarize the given text in a few sentences.

My girlfriend and her ex-husband were given a substantial gift toward the purchase of a house some years ago by my gf's mom.  As part of the divorce settlement it was agreed that the ex pay back his portion of that gift which he has done.  In exchange, gf and mom have agreed to compensate him for any tax liability he incurs after claiming a gift tax write-off.  It's come to our attention that he could write off the whole thing, as long as he's willing to have a couple thousand dollars count against his life-time estate tax write-off.  The life time limit is over $5 million, and he is very unlikely to approach that.  

Ex is being stubborn and doesn't want to write off the whole amount.  If we insist he does take it and don't pay him, would he have a duty to mitigate his damages by using the write-off.

Morally speaking, ex has been far from generous in many ways throughout the divorce process I know it's legally irrelevant, just want you all to know we aren't pretty assholes.
<bot>: gf will owe ex money, butonly if ex doesn't use a fairly painless tax write-off. Does he have any duty to use said write-off?