Are iPhone credit card readers safe?

I swiped my credit card into the iPhone reader of a little old lady who has had a soup kitchen baked goods stand set up all week. I have never used one of those things before, so now I'm nervous. Are they generally quite safe?

Added (1). It was squareup and I received a confirmation from squareup.com

You are fine, those are common, many use them as its just like a card reader to send the transaction, its not actually stored on the phone.