Two pressure pumps on a Megaflow system?

Our shower has a water pressure pump located in the bathroom. It's been working well for many years. During recent renovations we had a new Megaflow system installed in the cellar (Joule Cyclone Platinum unvented cylinder) with a Sauermann Si-83 power pump. When he was checking the installation the plumber turned on the shower, then he switched on its pump and a pipe behind the shower burst due to too much water pressure. Water poured into the room below. The contractor is charging me for repairs and a new shower. I questioned this as I believe the plumber should have known the shower pump shouldn't be used with the new system. The contractor's reply was: "please understand it’s not our plumbers mistake or fault as your existing gravity system mixer wasn’t designed to withhold the pressure in the pressurised system. Please note, that this would’ve occurred with any other plumber as we did our due diligence by surveying externally but ultimately, we had no access to internal mechanism and/or manuals to review and analyse. Moreover, you’d have to integrate into the new system anyways" I don't understand much of what he's saying but is he right? Was the burst pipe unavoidable?