My old Volvo 740 Estate needed over 40psi in its rear tyres when loaded.

In addition, in case of a rear tyre puncture when inflated to the higher pressure I kept the (full size) spare at that pressure.

Never had any problems.

The tyre fitter's advice is complete rubbish.

Inflation pressures are always specified for cold tyres, so 42 is less than 44 anyway but the "maximum pressure" marked on a car tyre's sidewall is not actually the maximum permitted inflation pressure. The reasons for this are difficult to explain to those without specialist tyre knowledge so I won't elaborate here, but a car tyre in good condition can survive pressures far higher than this "maximum pressure".......

The fitter should know that when fitting a car tyre to a wheel rim 3.3bar (48psi) maximum inflation pressure is initially permitted, and for final bead sealing this can be increased to 4.0bar (58psi) if necessary. Provided the tyre is undamaged, these pressures will cause no problem whatsoever.

For car tyre pressure settings in normal use, the tyre industry guidelines are a maximum of 3.2bar (46psi) for car tyres up to T speed rating, and 3.5bar (51psi) for car tyres with speed ratings above T. However these inflation pressures can be exceeded with the approval of the tyre manufacturer.

A car tyre in good condition will typically hold a pressure of at least 90psi without bursting. However, before this pressure is reached it is likely that the wheel itself will fail. Note - wheel rim limits are particularly important for plastic rims such as those fitted to wheelbarrows. Usually a limit of 2.5bar applies which is well below the tyre's limit.

With regard to car tyre "blow-outs", apart from a few caused by damage to the tyre (impacts etc.), they are the result of inflation pressures being too low due to poor maintenance, punctures etc. Low pressures cause the tyre to overheat at speed and it will eventually fail at one or both shoulders (these being the hottest running part of the tyre). The final outcome is complete detachment of the sidewall(s) and tread.