The only guidance is that in worksheet 1 of publication 501 and in tax regulation 1.2-2(b), the head of household has to pay more than 50% of the expenses.  So, if the expenses are being paid directly by them and not someone else, then they would qualify as head of household, especially if they have sufficient documented forms of income, such as child support (non-taxable) and spousal support (non-taxable if from post 2018 decree).