He'll need to pull some of the the floor up to find the leak, replace the insulation and to dry out the area under the floor. Yes, it can be replaced - it's probably plywood or more likely OSB type flooring. He will need to make the cuts with repair in mind - for example on top of the floor joists to have something to nail (or screw) back to. The floor covering (likely vinyl) will have to be replaced. The most difficult areas will be where the damaged areas crosses under a wall or appliance.
Since it is a "new leak" and has not had a chance to rot the wood, he may be able to dry the area completely after removal of the insulation and not have to replace floor joists, etc. Of course, any wood that is swollen or damaged will need to be replaced. This is not a time to scrimp on cutting the wood out - he needs to get access to allow proper inspection, replacement and drying.
Good luck, but it is repairable. I doubt you'll find a pumbing diagram.
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