A question for you all…….. when a Millionaire sees that you have spent a lot of time working on his faulty thermal camera and declared it beyond economic repair through quality diagnostics…. How much do you think the Millionaire pays the tech who spent that time on his equipment ? I will let you guess 
It was my fault for not sending a diagnostic time bill for payment as that might have pricked the chaps conscience.
You put these events to the back of your mind and move forward. They hurt at the time but life goes on 
I don't know your millionaire, but look at it from his possible point of view. Some repair shops charge for their time, as if they are consultants. If that is the game, everyone needs to know up front. Others charge if they are successful and can offer the customer some value, otherwise they view the cost of their time as a preliminary expense for which they bear the risk. I typically have chosen the latter principle. That doesn't mean I wouldn't charge for diagnostic time if that were discussed up front, but I would only charge for that time if either a) the customer was offered a repair solution and declined b) they had the repair done, in which case the charge is a combination of diagnosis and repair (note that often it is 99% diagnosis and 1% repair since for some things the 'repair' part is very simple) or c) the customer has an unusual or difficult issue that they want determined and they understand the possibility that at the end of the process there might not be a repair option and they are OK with that. What I have never done is take a job in with only an agreed upon diagnostic fee and then return it saying "I can't fix this and you owe me $xxx.xx.
So if your millionaire presumes that you operate like that and you didn't ask for money, he may not be such a terrible person. Myself, if I knew that you spent two days on it, I might offer to pay you even if you didn't ask. But in my (former) circles that would be seen as almost disrespectful on the customer's part--the presumption is that a business owner sets their own rates and the customer's obligation is to pay the bill promptly without complaining. Tips are for the lowest economic classes, waitstaff and parking valets.
In my case I won't accept money for repairing stuff, simply because I don't want that end of the business and I'm mostly a buy/fix/sell hobby anyway, not trying to earn a living. I will accept wine, single-malt (although I'm not offended by the uneducated that might bring me a blend) and interesting junk electronics as payment. You should set rates and stick to them--people that balk at paying you, including friends that know you are trying to earn a living, aren't people who you need to be working for.
There is another consideration to your story; the fact you were dealing with said rich person's PA, not that person directly. PA might have found you as result of a random Gurrgle search after failing to get any desired response from the MFR, then all this went down without said millionaire even knowing that someone who actually knows something spent days on it, and said PA was either unwilling to take responsibility or not authorized to pay anything for said work.
You never know.

Having spent 40 years on/off "in the repair game", in a number of different disciplines, one thing I've learned is to be firm with the customer in one thing:
"Diag work takes as long as it takes, period."My usual approach is to quote a minimum charge up front... explaining that my time has value (time is, IMHO, the
only currency worth considering, as it is nearly the only cost which cannot be fixed with money), and that
x dollars will buy
y amount of my time. If I'm able to diag and repair the DUT in that time then
Yay! you got out of here for just that minimum plus parts. If I reach the end of that time and I still don't have a answer then we can revisit the deal for more diag time.
Make sure you quote a number that if it is paid in a reasonable time without assache you will not feel taken advantage of, and stick to your guns. Don't fall for every "but I'm just a poor boy..." story that comes along unless you really want to; but realize that
you are responsible for that lost time, not the client. If it makes you feel better about charging for your time, pick a ticket every once in a while to do pro-bono. I've done it.
But
never feel guilty about asking for money for your time. You're worth it.
mnem
