Hi,
My partner recently repotted some very large pots of bamboo, it was a struggle to get it out of the original pots and he ended up with bamboo root dirt, dead roots and compost from the original pots all over the garden. My worry is this bamboo will reroot itself and the precautions we put in place in the new raised bed will be a waste.
How can I make sure it doesn't re root itself and spread all over the garden?

They won't grow from dead roots or dirt. The rhizomes are not tiny, and you would notice any big enough to grow. Don't panic.

@Hortica is correct about re rooting however bamboo can spread. Check to see if you have a clumping or spreading type. If you have it in a raised bed it can still send rhizomes under the dirt and spread. I would Google and YouTube 'bamboo care'. I have some beautiful green and yellow bamboo which is blocking the view from the second floor and has spread about 10 feet. Make sure you control it before it gets out of hand.