I agree with what Christy Lee said about not excusing her behavior, and I love Suz T.'s suggested language for dealing with the mom. I would add in that your daughter has been punished, not just that she apologized. I further agree with Suz that you are perhaps a little too inclined to blame another girl for the bullying and to defend your daughter for being the inclusive one. Face facts - most kids do bullying or hurtful things at one point or another, so you MUST stand up to that the first time, as well as the second time, and not say "Oh, this isn't my child." This has nothing to do with her being a single child (that's blaming yourself for not having other kids for her to rough house with), or her ADD (that's not why a kid pulls down another kid's pants), or her physical size/clumsiness (she was obviously quite adept at doing this, not clumsy in the least).
All kids talk about this prank stuff all the time, and this is a perfect opportunity for you to talk to your child about what happens when kids get in a group and spur each other on - "mob mentality" isn't just what happens with adults on the streets of the 'hood, you know! And talk about privacy and private parts - areas covered by a bathing suit are the other person's alone. And bodies in general have boundaries - people don't always want to be touched.
Talk to her as well as defining what's "funny" - if she or another child think it's "funny" to pull down pants or dye hair when someone's asleep or push someone into the pool or text/sext, what makes it funny? Are you laughing at someone else's embarrassment or pain? The definition of "funny" is when EVERYONE is appreciating the joke equally. It's hardly ever, maybe NEVER, when someone is having something done TO them.
Predicting consequences is a function of brain development - it is that part of the brain that fully develops around age 25. That applies to kids with lots of siblings and kids without a diagnosed condition. They ALL need this help from their parents. So you have a lot of years ahead of you, helping your child deal with learning to predict outcomes. Sometimes kids learn that "I'll get in so much trouble if I do X" much earlier than they learn "Gee, that would be cruel to do." Your job is to impart consequences and guidelines right now, and make sure you aren't excusing behavior because you don't see it in context without other siblings around your child.
You have recognized that what your daughter did is unacceptable, and you are contemplating talking to the other mom. I think you're on the right track. I don't think it's the job of the hostess, unless you don't take care of it yourself. Do the right thing and both you and your daughter will ultimately feel better.