You ANBU. Put your DD first, don't go.
Your DD is your child and if all goes well will be in your life for the rest of your life, families do fall out sadly but hopefully not yet or in the long term.
If these friends moved to the other side of the world you wouldn't be trying to keep your past arrangements going? If they got divorced would you try to keep the arrangement going?
It's an arrangement and it worked until it didn't.
It's sad your friends child is going through whatever it is they are going through but as sad as it is theres no reason to put either child or family into a stressful situation.
You child is the most vulnerable person in this situation, the one person (due to age) who hasn't got the life experience to deal with this situation - you have.
Why are you, as the adult and the parent even asking the question?
Trust your instincts.
You know going would be way too much for your child to deal with and will only stress you all out.
If it was to go for lunch or tea your DD would have the chance to gain some experience and skills for dealing with these type of situation in a manageable way. But it's not.
It's to stay in the other child house with no safe place to retreat to and for a longish period of time.
You already know the best thing to do for your family. Good Luck and I hope you have a good outcome for everyone.