Perhaps you are thinking it is something like HIV/AIDS but it can't be caught or transmitted so you dont need to be worried about that
Probably the hardest things to realise about lupus is that it is a very variable disease. It affects people differently because it is a complex multisystem disease, and the symptoms vary in severity from person to person.
It is not necessarily progressive and often waxes and wanes. You can feel more or less fine one day or for several weeks or months or even years but then it can surge into activity again unpredictably.
So you need to find out how your friend is affected by it and how it affects her life and learn how best to deal with those effects.
There are some "Letters" that give an idea of the sorts of problems that might be encountered, on the Family and Friends section of the forum. The Relationships and Living with Lupus sections also give insights into the difficulties we face in relationships.
Some additional understandings have to be reached when forming a worthwhile relationship with somebody with any chronic condition.
Good Luck
Clare