Hmm... well, if someone doesn't self-identify as having ADD, then sometimes they can actually take it as an insult if you tell them that you think they have ADD... so you are right, this is a very sensitive subject to bring up.
I am a adult woman in my 20s who has ADD, but I know it and I know I need to take medication and get help for my ADD or else I do suffer in college... and my relationships suffer too, and just my whole life suffers. But... for someone who doesn't think they have ADD or for someone who doesn't have ADD, saying that they have ADD can be hurtful for them to hear. For me... it's just like "yup! I know!" but... for someone who doesn't think they have it or just doesn't actually have it at all... they could get very defensive about it and take it as an insult.
But... if she is experiencing symptoms of any sort of disability, included ADD... and those symptoms are getting in the way of her functioning ability (sometimes theses things don't get in the way of people functioning ability)... then it might be good if she had support and help. A visit to her family doctor might be a place to start. Or, a visit to a counselor or psychologist.
If she was still IN school... then, you could always suggest that she visit a school counselor if she was struggling in school... that would probably be the best place to start. But... it sounds like she is not in school anymore. But, you talk about things like rent... not paying rent can be extremely stressful. If she ever comes to you to complain about financial stress, you could suggest that she see a counselor (and then refer her to a psychologist, as they are licensed counselors who usually understand the medical stuff behind things like ADD) to help her manage the financial stresses in her life.
Mainly, just have support services ready if she comes to you for help... such as information on where she can go for help, such as her family doctor or a counselor/psychologist.