If you and the friends do all four disney parks and Sea World your trip will be full. A 9 mo old should be happy with whatever the rest of the groups wants to do. The baby will love all the lights and sounds no matter what park or activities you choose. She can go on any ride without a height restriction which is a lot. She should be fine in almost any resturant. Your, or at least their ,touring style may need to be slower paced and adjustable to the baby's needs.
Suggestions to them would be to bring thier own stroller and a baby sling to carry the baby onto the rides.
A onsite resort would be ideal to allow them to return for a nap or break without you. It will be fun to see WDW though the eyes of a little one.
Jordan's mom