Who gets the Pet in a break-up?

Have you ever been in this situation?  I was having cocktails with a friend and her and her boyfriend decided to get a dog.  This will really be his dog because she already had a dog before the relationship. 

Background:  She was living with a ex-boyfriend for many years who also owned a dog.  When he moved out, her dog showed signs of separation.  She was also a bit sad not being able to see the other dog again. 

Now that she is living with someone else, he has decided to get a dog and she can't help to be excited  She thinks this will cheer up her furkid.  My friend decided to lay everything out on the table at the adoption agency.   They discussed if they were to break up, she will take custody of the dog.  Since he works out of town a lot, he agreed.  They proceeded to sign adoption papers.  Some of her friends thought it was harsh.  I thought it was perfectly acceptable to discuss something so serious before commitment. 

Custody fights over pets have become more common over the past decade, and cases with lawyers are up 23 percent in the last decade. "People will kill the pet rather than have the spouse get it," says one divorce lawyer. –CBS Sunday Morning

What do you think?  Do you think he will honor this if they decided to break off things years down the line?  What would you do? Should this be a part of a pre-nup?