Client has 2 children but let partner claim them as dependents and file HoH using those children. However, she pays the childcare and he did not file for that credit. 

I was under the impression, from what I have read, that it was possible to use form 2441 for child care even if another person claims the kids as dependents. Is the problem that he is using them to file as HoH? 

i did not anticipate this when I advised her on how to do this, but from what I read it should have been fine: 

https://ttlc.intuit.com/community/tax-credits-deductions/discussion/can-i-claim-child-care-expenses-...

https://www.irs.gov/taxtopics/tc602

This return has been rejected by the IRS, because one or more qualifying persons identified on Form 2441 Part II, Line 2(b) has already been claimed as a qualifying person on another taxpayer's return. To identify the qualifying person in question, please review the "Description" field above.