Hair tangles so easily??

My friend has super tangly hair, and we can never get it to stay smooth for more than 10 seconds. She has super thick, long and straight hair. I was brushing it for her, and I did it in sections. I put the tangle-free sections into hair ties, and the minute I take them out again it s tangled. How can we prevent them from becoming tangled so easily?? Are there any good products out there?

Thanks!