Sorry...but I'm siding with your friend on this one.
I have two children. I breastfed them both, and at that age, all they do is eat and sleep. I didn't start leaving my baby with anyone until they were at least 6 months old. Even then it couldn't be for more than an hour at a time, as I was still breastfeeding full time. Everywhere I went, babes were with me. It was either that, or not go. Just the way it is for a lot of mothers.
To be honest, if she's a good friend you should let her attend with her baby. You probably won't even notice the baby there. Most newborns will sleep through everything and anything (I know mine did...the noise would actually lull them to sleep).
I can completely understand where she is coming from. Her children come first and foremost.
Also, it may not be worth the time to get out of the house for just 30 minutes. It's a LOT of work when the baby is that age. Your friend is going to be EXHAUSTED. Mothers get virtually no sleep at night with a newborn constantly waking up to eat. When my kiddos were that age, I was almost 24/7 in sweats and a tee-shirt...and unless we were going somewhere WITH the kids, I'd send my husband out to do any and all errands. I was just too darn tired. As most mothers are.
You're going to have to be a little more understanding in this situation. It's a newborn, not a loud rioting two year old child. That's fine if you want absolutly no kids at the wedding...but you need to be more understanding of your friend. If she can't come, it's becase she's being a good mommy, and putting her child first.
To those of you saying that the friend should just 'get a pump, and leave expressed milk with a sitter'...well...this just DOES NOT work for a LOT of mothers.
My two children BOTH refused to take the bottle. One time, my mother (who was babysitting our then 1 month old daughter), actually had to feed her with a dropper (like the kind that comes in medicine bottles), because she outright would NOT take the bottle.
Sorry, but for many breastfeeding mothers out there, bottles are just not options. On top of that, if she is exclusively breastfeeding, giving a bottle to that young of a baby is NOT a good option. It causes nipple confusion, and can have severe consequences for a breastfeeding mother.