I cannot believe that a plumber didn't understand that most kitchen cabinets (except Ikea ones) have a 50mm service void at the back and a 150mm high plinth height to work under. He must be the one in Russ Abbott's "Plumber's Song" where the singer refers to turning up "with some half wit that he's trying to train"
Seriously though, it should be possible to fix relatively quickly if your builder pulls his finger out and treats it a bit like an underpinning job - in other words props the floor above, puts in some extra timber packers into the gaps in the blockwork, then knocks out the blockwork on a hit and miss basis, replacing a block, leaving a block, replacing a block and so forth (starting with the worst damaged ones), then after the mortar has set in a couple or so days doing the same again with the remaining damaged blocks. We had a clown who did something similar with CH pipes on a couple of single skin brick walls a few years back and that's how we "cured" it. Cost us a week on our schedule