I am in a similiar situation, and can tell you from my experience, DEFINITELY send the card.
We had a preemie born at 25 weeks 18 months ago. A few weeks after she was born, a co-worker of my husband’s delivered her baby at 26 weeks. We were told our daughter would not live more than 2-3 days. We were told IF she did live, she would never come off the ventilator and never leave the hospital. My daughter is a living miracle. Not only is she alive, she is off the ventilator, home, and doing great. She is still on oxygen, has chronic lung disease and some other mild medical issues, some mild developmental delays, and is very small for her age, but SHE IS ALIVE! I have become very close with the other mother. Her daughter was seemingly much healthier than mine initially. Unfortunately, she had a much harder time coming off the ventilator, ended up with a tracheostomy (hole in her neck to permanently connect her to the vent), never left the NICU, and died at 8 months old, 7 months ago Thursday. I am in contact with this mom nearly every day (so we are closer than you are with your friends). Her biggest fear is that people will forget her daughter. She has told me time and time again that she wishes more people acknowledged her by name, and she gets very very upset when close friends and family do not acknowledge “milestone” dates, such as 6 months from when she died, her original due date, her birthday, etc. Believe me, by sending the parents a card (and even something else small if you can) you are most definitely NOT reminding them of their child. I can almost promise you that child is on their mind every single day. But you are reminding them that you cared enough not to forget…and that says a lot!!! You are an incredible friend for that!