With only one fan extracting air and no fans pushing air in, it is little wonder you were over heating. The poor old CPU fan on the motherboard was working overtime just to keep the CPU temps down as it had very little cool air to work with.
I haven't seen a CPU yet over 1Ghz that didn't benefit from a rear case fan. Fans which don't come as standard.
I would install another fan at the rear just bellow the power supply fan blowing out and a fan at the front sucking air in. I think you will be surprised at the result. It will take some of the pressure of the CPU and allow it to slow down, that assuming that it is capable of being speed regulated which not all fans are.
I would avoid that "pc engineer" as well. To replace motherboard, CPU and HDD without first trying a couple a fans is a bit of overkill. But then he now has a perfectly good motherboard, CPU and HDD that you paid for.