My daughter's doctor is 99% sure she has mthfr defect. What can you tell me about it?

Depends on which one. Methylene tetrahydrofolate reductase (mthfr) mutations, deficiencies, & other abnormalities are associated with a variety of health issues, including clotting disorders, congenital heart & aorta defects, metabolic disorders, B12 deficiency & autoimmune atrophic gastritis, cleft lip and palate, & miscarriages. Her doctor can discuss which specific mthfr problem she has and what it can cause.
Test. Most labs such as quest and labcorp can do the mthfr test. Prices can range from $120 to $400. About 1/4 of the population has some kind of methylation defect, so i think it is worth checking.
Folic acid chemistry. Homozygous mutation at either the a1298c or c677t location significantly decrease methylation, the process by which genes are regulated, neurotransmitter balance is maintained & consequently mood. The c677t variant is more easy to support by bypassing mthfr with supplemental 5-methyltetrahydrofolate. The a1298c variant is more complicated because of the way the mutation affects the mthfr protein.
MTHFR Gene mutation. If you'd like to do more reading on the health effects of mthfr gene mutation, here's a great article ...