Depending on the culture in your country, a sympathy card could be a good thing to give. You might find a card that says something compassionate, then add a personal note expressing your condolences and offering to be a listening ear at work or outside work, or if they’re looking for a distraction you can be that as well. Something along those lines might be appreciated. It of course depends largely about the kind of relationship you already had with the coworker, and what kind of time you’re willing to offer her outside of work (if her mother had been living with her, for example, she might need someone she can call in the evening to talk when facing that loss).