I'm going to tell my pregnant friend she needs to focus more on the baby she has already got
Why do you feel the need to tell her to focus on one child?
She's spent money on buying stuff for the new baby, she can't afford to waste that money.
Friends should be there for each other during important and difficult times. Sometimes it takes a real friend to tell another their concerns about their behavior. Just try to do so in a respectable manner in order to not upset them during their pregnancy.
She wants to spoil the new baby. She's a terrible mother.
It is good that you are concerned about your friend's older child, and you should be ready to let her know when she is getting too distracted from her responsibilities.  However, you should not judge your friend as being a terrible mother.  For all either of us know, she is trying her hardest to improve as a mother, and she might be willing to show as much care to both children.