Custodial parent is considered HoH.  Other parent is single with a dependent due to a separation or divorce and must have the custodial parent provide form 8332 releasing the dependent child to them for tax purposes regardless of what the divorce decree says.

I always have my clients provide me with a calendar showing which nights they had the child with them and then go through the tie breaker rules.  On occasions had to do a paper return with the backup attached showing they were in fact the custodial parent.

If your client had the dependent 51% of the time, then your client is the custodial parent.  If is it 50 - 50 go to the tie breaker rules.  It can get messy between the parents.