Good day fellow forumers!

I've hired a contractor to do renovation for my parents' 2 storeys old house.
I've asked them to totally knock down everything inside the master bedroom toilet.
And I've asked them to change new wiring for the (instant) water heater shower unit too.

So, just now I went to check the progress and I found that the contractor used 2.5mm for the live wire,
but only 1.5mm for the neutral and ground wires. And they tiled it up already. Is it dangerous?
Should I ask them to knock down the new tiles and redo the neutral/ground wires with 2.5mm ones?

My parents' house is on 3 phase power. And there is a sub DB upstairs powering 3 bedrooms.
The said downsized wires are upstairs in the master bedroom toilet.

Thank you experienced sifus for your inputs!!

Understood, thanks lj0000, ceo684 and Selene Yeo for the advice!! Sorry for late reply, was busy juggling work and renovation.
The contractor basically just pushed the blame on his staff. Excuse is 2.5mm finished, so his staff used 1.5mm. Quite BS la.
I warned him to be more careful and professional.
By the way, I've also decided to change completely new wiring for the common bathroom's water heater from DB too.

I bought Mega 4mm wire for the contractor to use this time.
The (instant) water heater I bought is Deka 4.5kW high power version.
For the RCBO, I bought Schneider Easy9 25A, 1P+N, 6kA, C Curve 10mA.
Schneider RCBO somehow doesn't have 20A version.

1) Should I buy EPS 20A 2P 10mA 6kA RCBO Type AC instead?
2) Why EPS RCBO has 2 poles, while Schneider RCBO only has 1 pole?
3) Is RCBO only good enough? Or should I use MCB + RCCB setup?

Thank you all again!!

my house using 2.5mm wires with 10ma RCD, not connecting to MCB. it means no protection on the overloading ?