There would be nothing wrong in taking your DM food and putting it outside her door but often poorly people are not well enough to get out of bed. That may help if she was a bit unwell but what if she is very ill?
I have a relative I buy food for and take it into their home and put it away. I look after their medication and monitor they are taking it and change the bed etc. You are allowed to go into the home of someone you are caring for.
If my relative did become ill be (unless only mildly ill) I would go and collect them and look after them in my home whilst taking precautions. I could not leave them with, say, breathing difficulties, in case they deteriorated and weren't in a position to call for help.
What you do depends on your individual circumstances:
Whether you have a family - a partner and/or children who need looking after. Do you have siblings who can help?
Whether anyone in your household is vulnerable.
Whether she and you have spare rooms in your homes.
What they are like - anxious, stoical, independent, needy.
Does she have a thermometer to take her temperature?
Would she use facetime/Skype/zoom to communicate with you so that you can see any deterioration and get help for her?