Your gratitude is, without a doubt, very much appreciated (But Not Really)
If you want to save time, it’s usually a good idea to agree with the other person, even if you disagree with their assessment of the issue or know it’s correct. This is because deciding with the other person can help avoid unnecessary arguments.
In this particular instance, however, it is abundantly clear that such an approach was fruitless; could you please explain why he didn’t trust the customer who reported the faulty pump? I don’t see any reason why I shouldn’t believe her.