As requested, I met with owner
In the rest of his letter, he proposes to conduct a water test to try and cause the leak to happen and only then will he be able to propose a repair to the leak and offer a guarantee for the repair.
Now, I tell hubby, I feel much much better because we're finally dealing with someone who (seems to be) is objective and does not start with the assumption that my window is the cause of the leak. Now, if after this test, it does come out that my window is to blame, then I have no issues about bearing the cost of the replacement. An expert opinion was all that I wanted and have been asking for all this time.