How is it not your fault when you make a joke that offends your friend? You said a joke that many people find offensive without seeing if it was okay. If you’re not close enough with someone to know that their mom died, you’re not close enough to make jokes like that. It sounds more like an acquaintanceship at that point. You are responsible for the things you say. This “it’s not my fault you were offended” applies to audiences of comedians who don’t have personal relationships with them, not friends telling offensive jokes. I’m glad you apologized but it doesn’t seem sincere when you’re thinking you’re not at fault.