I would start by asking how many water tags were turned? Even if there is a single one turned I personally reject the machine as over time the corrosion can migrate and stop a motherboard in her tracks. The machine was working as you did your work and virus protection tests. That heated up the machine and could have caused the traces that were corroding to fail.
The only thing I would do at this point is pull the replacement drive and put a known good one in and test to see how it runs. If it ran right and you could load an appropriate system and updates without further problems, I would add the extra time to the bill as well as the drive cost and wish them a wonderful day. The only thing I warranty is the drive I put in that they bought from me and the time for it. Nothing else as I have no idea where the parts came from and I didn't make a dime to offset any time I may have to invest in the future.
I don't know why one would allow a client to bring their own parts to a repair as we develop relationships with our own venders of trust. I also never let a client watch me work on their machine as it tends to only have a negative outcome from their perception. Liquid spills are like a rabid dog. You never know when they will strike.
Good luck but don't be bullied into something you will regret down the road.
Yes I know you know everything I wrote but it is just a friendly reminder that you most likely did everything physically right but stuff happens outside of our control especially on spills.