Yes, it is usual for the seller to pay the buyer's real estate firm's commission fee but sometimes (particularly if a home is a short sale or a for-sale-by-owner) the commission is not offered by the seller.
Hopefully, your friend's agent should have informed her about the possibility of this extra cost upfront so that it wouldn't be a surprise to her later in the transaction.
Prudential Connecticut Realty
I'm not sure why she did that? However, if she agreed to it and signed something to that effect, then she did. Usually, the seller pays all commission - for both the listing agent and the buyer's agent.