I have always considered that the parent that is earning 70k has paid more than half of the child's support, which then disqualifies the other parent under the tiebreaker rules  - I know that's super conservative approach, but my experience has borne this out. 

I've told clients to go to another preparer and have them do their return in such a case. Not worth the hassle. I just don't see how they can make the case the lower income taxpayer who makes 15 percent of what the higher taxpayer makes is paying over half of the child's support.