这里的“单独一个人”可以有两种说法：alone或by herself.alone作为副词使用.
"大卖场"可以译为hypermarket,supermarket,shopping mall等等.
整句话可译为：
My mother often goes to the hypermarket by herself.But now she is shopping with my cousin!
或
My mother often goes to the hypermarket alone.But now she is shopping with my cousin!