Hi B.,
I too have a friend that has a baby boy with a heart condition and Down Syndrome. When I heard, I immediately wanted to know more about what this family will be going through. I bought the book "Babies With Down Syndrome, A New Parents Guide" Edited by Karen Stray Gundersen. It has many chapters written by different doctors, therapists, and parents etc. I found it to be very helpful.
Chances are her baby with DS isn't any different than any other new baby. Sleepless nights, tired parents, dirty diapers etc. Bring them dinner, hang out with her, offer to come over and hold baby while she showers... just like any other new parent. Don't be afraid to ask questions. Chances are, she wants someone to talk to like any normal parent. Ask about his birth, let her tell the story.
As the days go by, he'll likely have a heart surgery (or two). He may not. He may be delayed in his ability to hold up his head and he may not. He may have a delay in his eating solids, he may not. There are a lot of unknowns with a child that has DS as the severity is yet to be determined. My girlfriends son is now 16 months old. He's had 2 heart surgeries and spent many many months in and out of the hospital. We have helped them financially, we've helped care for their older child, helped house sit and dog sit while they were at the hospital days on end. I pray her baby never has to deal with all that my friends did (and still have ahead).
In any respect, just let her know that if she needs anything, you are there for her. Whatever it is... That goes a long way! Tell them CONGRATULATIONS on their beautiful baby boy! Having a child is a wonderful thing. ALWAYS!