Firstly, this forum does not give advice on gas/combustion matters or encourage DIYers to open the combustion chambers on boilers.
The DC resistance of most boiler type fans is round about 40 ohms. BUT the DC resistance is only one aspect of a fan motor and unless its open circuit unlikely to indicate any fault. Other problems can stop a fan turning at all or turning at full speed.
I don't think that Ariston should have been speaking to a non gas reg person at all! But if they did agree then they should at least have given you the correct resistance value.
The correct way to repair a boiler is for a gas reg engineer to diagnose the fault. Not to just replace parts without any good
reason!
As agile said you will get more complete answers on the combustion chamber forum, Your GSR qualification will be question by the powers that be before you are allowed to put the question even, and that can take a few days which your girl friend may not like.
Also I have spoken to the techs on same boiler and they don't know what they are talking about half the time so don't take their word as gospel.
Tony