Taking ownership of Characters

I have a campaign that I’m the DM and none of the players are active anymore and I would like to take ownership of the characters. My friend initially started the campaign with characters made on his account, but the campaign and characters are gone from his account. Is there a way to take ownership of them? I am not given the opportunity to unassign the characters. 

No, and it would be bad if you could. Only the character owner can unassign the sheet so that someone else in the campaign can claim it.