the system will burst under pressure, this pipe is the relief line for the cooling system, it is a very important part of it, The way it works is that when the engine gets warmed up the coolant water tries to expand (heat) the radiator cap large heavy spring maintains the rating printed on it, meaning it regulates the pressure in the whole system at 0.9 bar.
If you block this pipe the pressure will never be controlled and something will burst open from the pressure.
Repair it correctly - I know its expensive business but you will feel very angry when you lose your engine to this little pipe - thats why Im telling you to go talk to the mechanic.