you cant ask them how they feel, you yourself know how they feel. even by thinking of trying to put urself in their position, you can sense what you would feel. i just feel like asking, isnt the best; if i was the person, i'd be like, wait, they really dont understand what they did? how dont they know what im feeling? Obviously im feeling this way!
You urself said its heinous. The persons gonna be feeling one way likely about it. The best thing to do is put urself in their position. Its hard i know, but think for a second, if you were them, what would u want out of a possible apology? If you are sincere and genuine and you did it in the way that you did, show them. tell them.
words can uphold so much value, use them wisely to make the other person feel what you want them to feel, which is you're sincere apologies and sadness over what you did. empathize and sympathize with them and how there feeling, comfort them.
Actions speak louder than words, kind gestures, reach out to them irl. i feel like speaking irl over text, is much more powerful.