It's not really weird, your cat was 'protecting it's territory' and saw the other cat as an 'intruder'. You can't really make them get along, but over time they might get used to each other if they're introduced slowly. If you put your cat or your friends cat in a separate room with the door closed, they might start to get acquainted by sniffing and pawing under the door. If that goes well, then you can let them have some supervised time together.