YANBU - I'm shocked actually that people think YABU.
I had chicken pox as an adult and it was awful. I had over a month off work and wouldnt wish it on anyone as an adult. However, I have had the opportunity to see friends whose children have chicken pox but I have deliberately NOT exposed DDs. DD2 was only a few months old at the time so why would I get DD1 to catch it and then affect her tiny sister. Next time I had a friend whose DC was having chemo so again I wouldnt choose to let my DD get it as then we couldnt have visited my friend when she needed us.
As it happens I posted in chat last night as I think DD1 came down with it yesterday - I am waiting until the morning to see
Its all very well saying 'but they'll get it at some time' but as in your case where you have an immunity compromised friend who you now cant see then I think it was irresponsible of this woman to not tell you that her child had been in contact with chicken pox.
I have no idea where my DD has caught it from but I am telling all of our friends so that they can be ready.
Hope your child is feeling ok.