When a friend's husband was diagnosed with cancer, I remember asking "What can I do to help?" Her response was "Just call and check up on us every couple of weeks." We no longer lived close enough to each other that we could just drop in for a visit. Then as now, I was not in a position to make a physical difference. Fortunately, making a difference to a caregiver does not require an investment of a physical or monetary nature. Instead of stating "If you need anything...", the caregiver would benefit from us reaching out directly.
If you live remotely from your caregiver, you can help by simply placing a call and saying "I was thinking about you today..." or doing the same in an email. If you come across an article or story in your reading that is related to an interest the caregiver has or the condition of the person they're caring for, you can share the article or story with the caregiver. Simply reaching out to the caregiver in this way lets the person know that the current focus of their work has not removed them from your thoughts or interest. It allows the caregiver to feel included in life outside the demands of the care they are giving.
If you live nearby, you can additionally swing by for a quick visit, offer to pick something up from the store for them when you're on the way, or see if you can treat them for lunch. If you were to pack a lunch and bring it to them, it is an inclusive act which is a dramatic change of pace for the care provider even if it is a small action from you. While these ideas do have a physical or monetary component, you can still fall back on the phone call or email.
If you visit in person or by the phone, listen. Ask "How are you?" and then stop. Just listen. Let the care provider vent. You may not be able to change whatever problem is vexing the caregiver at that moment. The caregiver may not be asking you to fix anything. Your gift in that moment is just listening.
Letting the caregiver know you care and letting them feel supported and less alone takes very little effort or money. It mostly takes a little time.