Dude, changing a sh!tty diaper on a table =/= breastfeeding.
I gather a few things from the OP.
1. That you may not be entirely okay with BF, otherwise, why would it have bothered you? If it was bothering your guests, and it didn't bother you, then you may have been more inclined to say "Well it's my house, it's no big deal" and left it at that.
2. Something tells me that you are exaggerating the scenario greatly.
I say #2 because no BF mother I have ever been around just takes her top off and exposes herself like that. Unless they are merely weeks after giving birth and are still learning how to BF. It is common for a BF mom to feel both breasts to remind herself which side baby is due to feed on, but taking her clothes off in a room full of people and looking at them?
BF takes practice. Sometimes babies pop off and mom is caught off guard. It's more important to catch baby and catch any spit up than it is to worry about a little nip exposure.
I think there are 2 ways to handle this in the future. One, distract your guests, however immature they may be, with a game, a gathering outside, or with a television show.
or two, offer her a blanket to cover with.
or three, make new friends. Honestly, being "horrified" and "gawking". Really? Are you friends with a bunch of junior high kids or something?
What is "normal" varies from mother to mother. Some women have no issues BF in a room full of people. Others will do so discreetly, under a cover, and some will leave the room.
All in all, it's just a breast. It's a baby who needs to eat. It's a new mother trying to learn the very overwhelming task of BF and tending to her baby's needs. Therefore, she needs your support, not your judgement or your "concern" over what your friends think. Because, honestly, if it truly didn't bother you, why would you be the one to say something to her?