So sorry you experienced a problem with the site.
We already know and can show that the daughter is not a qualifying child because of age and she is not a student.
That leaves relative. Her income knocks her out of being claimed as a qualifying relative. If you show them the IRS publication 502 (easier to read than IRC) section that reads:
"You can include medical expenses you paid for an individual that would have been your dependent except that:
1. He or she received gross income of $3,950 or more in 2014,
2. He or she filed a joint return for 2014, or
3. You, or your spouse if filing jointly, could be claimed as a dependent on someone else's 2014 return
If the customer is not satisfied with claiming the medical and still wishes to claim the daughter as a dependent then you would have to walk them through the tests for dependency.
A qualifying child is a child who:
1. Is your son, daughter, stepchild, foster child, brother, sister, stepbrother, stepsister, half brother, half sister, or a descendant of any of them (for example, your grandchild, niece, or nephew),
a. Under age 19 at the end of 2014 and younger than you (or your spouse, if filing jointly),
b. Under age 24 at the end of 2014, a full-time student, and younger than you (or your spouse, if filing jointly),
or c. Any age and permanently and totally disabled,
3. Lived with you for more than half of 2014,
4. Did not provide over half of his or her own support for 2014, and
5. Did not file a joint return, other than to claim a refund
Their daughter clearly is not meeting the test because of her age and not being a fulltime student.