If no such thing exists in the DR, this could be a problem, not with USCIS but if it is at interview, she'll need to explain it honestly and directly, and the consequences will be what they will be.
There are countries that actually have civil union or domestic partnerships but the DR is not one of them. Neither is recognized there.
Actions have consequences. You may will have to wait about 18 months after filing to find out what this little deception's consequences will be, if any.