My question involves a marriage in the state of: OH

Sorry, did not know where to post this...

My friend has been separated from her ex-boyfriend for over a year. They purchased a dog together while living with each other. They have shared custody of the pet. Recently the ex notified her that he does not want to go on "sharing" the dog anymore.

What can be done in this situation? Here are a few facts.

They split the cost of the purchase of the dog (she's the one who drove to pick it up). They split the cost of food and vet bills.
Her children really love the dog and from what i've heard the bond isnt that strong with his kids.
Her schedule is more open to giving the dog its proper attention and care and when she works her children take care of it. He has a multiple-day on/off schedule which often leaves him giving the dog to her while he does multiple day shifts in a row.

Any thoughts on this?