Sure - If she were a famous historian, I am sure she would qualify for the HSMP - Highly Skilled Migrant Progam, and be able to secure a visa to come and research her heritage. But the Key words being HIGHLY and SKILLED, generally don't apply to 18 year olds.
If she wants to come over and spend 6 months hanging out in your local library researching her parents, she can do that on a tourist visa.
Seriously, we aren't lying to you. It's just not that easy to come over. The UK (and all other countries) have laws in place to restrict immigration, however unfair it may seem, it's the reality.