That depends on what "breaks". If he just soft-bricks it, you can find help oon the internet to fix it. If he hard-bricks it, it's usually possible to get it working again, it's just not as easy.
But if he causes hardware to go bad, the usual "repair" is to buy a new phone.
It also depends on the phone. If it has a locked bootloader (and does he even know what that means), does he know how to unlock it? Is that phone's bootloaded capable of being unlocked? (In some cases the answer is "no" which means that if you want to flash things you buy a different phone.)
If he has the same model phone as you do, and can restore it to stock - then show you, right in front of you, rooting and flashing it, I might trust him. If he can do it 3 or 4 times in a row, I would trust him.
BTW, since most ROMs [that's what you flash] ARE rooted, someone who says that he can root AND flash your phone sets off small alarms in me. Flashing a rooted ROM leaves your phone rooted without doing any "rooting".