My cat got out and my friend caught him, but I don't want to thank my friend.
Oh really? Why don't you want to thank him?
I might insult him because of his deed, I am worried he will expect a sincere thankfulness from me
I'm glad you got your cat back! You should thank your friend for catching the cat for you though. He shouldn't be insulted by that!
Thanks, he didn't do much.
Even if he didn't do much, he did something and still provided help.  It's even less of a gesture to simply say thank you, so it shouldn't be hard to say and your friend will feel appreciated.