Give them one or two hours of your time so that they can get away to do shopping, go for a walk or anything to give them a break.

Encourage them to take care of their own health.

Involve others who may be able to offer their time to provide a break for the caregiver.  

Offer to dust their home or run their vacuum.  Maybe do their laundry.

This is called respite.  One step at a time can be a huge relief for the caregiver.  It helps their physical and mental health.

Make a date to pop over for a cup of tea and just talk.  Let the caregiver do the talking.  This is for them.  Or don't talk at all.  Just be there for tea.

It's the little things.