Oh poor you and your friend.
I don't think there is a right or wrong answer there, but pointing out the bright side is often done for the benefit of the person talking rather than receiving and it leaves us feeling misunderstood or not having permission to wallow in misery which we do ocassionally have to do.
It's like handing someone a tissue when they are crying. You're often really saying 'stop as quick as you can please' when the person might really need to feel her tears drip all down her face.
If you're a good enough friend to be posting here, you're probably good enough to do everything right. Honesty is the best thing and if you don't know what to say or think your words might be wrong just say so.
Honestly, it is the spirit with which you say them that is important and if you get that right then your friend shouldn't mind pointing out anything that makes her feel less comfortable.
Lots of friends say my ds is autistic. Some of them I think 'grrrrr, he is a child WITH autism' but the others I probably use the same terminology with them because he truly is accepted as he is and they are just words.
You can't ever reassure her that her daughers condition might not be so bad, but you CAN support her in HER desire to have the highest expectations of her daughters ability and of the provision she should get to support her.
You can also help her by TRULY celebrating in the successes. The smallest achievement that you take for granted with your dd will light up her life and she might want to talk about the day her child made a very small movement over and over but unable to because very few people will understand the potentially life-changing possibilities that come with being able to move just one finger.
You sound lovely, and no-one like you should every feel you shouldn't post here. The fact that you have done makes a difference to a lot of us.