M.J.
I think since there is a vaccine now, many people don't remember what the chicken pox were like or how they get transmitted. They are airborne and you are most contagious BEFORE the spots show up. Two of my kids had the pox and every year the school nurse calls and asks me about the vaccine since it's not on their physical. Luckily I took photos and offer to bring them in.
I wouldn't be offended by your friends suggestion. It would make me thing WT--- (fill in your preferred phrase). Then I would cut her some slack and think that she probably thought she was doing me a favor offering to have my child exposed.
I understand your concern about taking your second baby out, but you really can't stay inside all the time. You just need to do what you can to keep him/her healthy and in healthy situations. Since you are having your next one in December, I would be more cautious than if it were summer since flu and colds are more abundant during the cold months.
I'm sorry you had to see this side of your friend. Unfortunately as your children get older you will see many things from other parents that will make you think WT---. There are some real "winners" out there!
I hope this helps.
M.