Does anyone here know what kind of knife and what method of attachment would be preferred for his type of work?

A fixed blade knife might be best for one-handed use, but I could imagine a fixed blade knife accessible to both hands could be very much in the way of work.

A custodian at my place of work had a finger cut off of his right hand while working alone on a weekend, and he simply could not reach his car keys in his right pocket to get to the hospital. He was able to dial 911.

If around rope and water, think the type of knife that'd be best is a canyoneering knife.

CRKT used to make a popular one, the Bearclaw. Hooked and serated blade with finger hole.

https://wildskyguides.com/gear-review-crkt-bearclaw/

Eldrid have something similar.
https://edelrid.com/gb-en/sport/climbing-accessories-shop/rescue-canyoning-knife?variant=194

And mounting it on your chest.

A fixed blade in the center of the chest is my preferred option. An alternate option I use is a fixed blade on one side and a folding knife I can open one-handed on the other.