Ask him to check the valvesprings. I don't know what's inside that motor but my 438" Street Hemi sees 7000-7500 with no problems. I'm not so sure you hurt it, it may have been wounded or weak already, but the water you're in could be treacherous, so to speak, and maybe paying 1/2 of it or more would be the right thing to do. Either way, the problem needs to be found for the your sake, his sake, and of course your relationship. There's darn few people I'd let drive my car...