When do you talk to husband's employer when he isn't officially diagnosed with FTD, but could make costly mistakes building a house?

How do you know its FTD if he hasn't been diagnoised. I think first, you need a confirmed diagnosis before you talk to his employer. Once that is done, telling his boss is something you can discuss with his doctor.

It’s the employers responsibility to access the abilities of all of their employees, not just the older ones.
I had a speciality construction company and watched all kinds of people make expensive mistakes.
If you rat your husband out with a disease that hasn’t even been diagnosed by a professional specializing in diseases like FTD your husband might be without a job earlier and home with you longer.
My uncle was far down the Alizhiemers path but was very fluent in his area of expertise. It was unbelievable that he could become so articulate.
For heavens sake let your husband go to work for as long as possible.

To tell or not to tell.

Just know costly mistakes can bankrupt a company. Yes, I've seen it because I used to be in this kind of business. The employer is responsible for not one person, but all the people who work there, and whose livelihoods depend on the company's survival.

What kind of testing has your husband had? FTD is a degenerative disease and evidence of this for sure will show up on scans.

My husband worked for his family’s business from the age of 16 until he was 63. After a stroke in 2003, he began to mess up, a LOT and I understand he cost the company a lot of money and made many mistakes. Finally, when he became incontinent (both ways) he had an accident in the men’s room of the company and for some reason never told anyone or cleaned it up, just went back to his desk. His family was incensed and called a family meeting which I was asked to attend as well. It was awful. They were fairly understanding because they knew he had issues, but they cut him down to half days. When he was hospitalized for a near-fatal heart infection, they fired him. We’ve since cut off contact with them.

The moral of this story is to please not leave it until your husband has to be fired by his company. Save him hurt and embarrassment. My husband internalized his feelings about being fired by his own twin brother, but I know he was and is devastated.