I am trying to find best way to help my mother during the COVID19 pandemic. She is living in senior rent controlled apartment. Any advice?

Try to appeal to her sense of patriotism and morality.  That if we all voluntarily self-quarantine, the sooner this will be over.  That people are losing jobs and hours at work, and many are having a difficult time.   That we have to care about others and everyone needs to do their part to get this over with as soon as possible.

Remind her what the government does for her -- the senior apartment -- and that we are all part of the community. 

Does she realize she is at high risk? The COPD has already compromised her lungs?

Really, she is probably stubborn and will do what she wants.

If she is still smoking with COPD she clearly doesn't care about her health.

You can not force your mom to stay inside. If she fully understands what is going on she is making a choice.
Rather than arguing about what she is doing because you probably will not get her to change, encourage her to follow safe protocol.
Wash her hands when she returns home.
Keep a safe distance from others.
Wash her hands often, and after touching objects. Carry hand sanitizer to use when she can not wash her hands.

I would also tell her that you are going to put your cleaning on hold for a while.
Drop off food and other things she needs but leave as soon as you can. Tell her that since she is putting herself at a higher risk you can not take the chance of bringing anything home to your husband.

And a side note...with the smoking and the COPD I do hope she is not on oxygen!