My husband is clearly losing his short term memory and physical ability to walk. Do I warn an old friend who hasn’t seen him in a long time?

Me too.

Forget that vow never to place him in care. Your new vow should be to consider everything that will keep him safe and comfortable. His illness can be better handled by professionals at some point. There’s no sugarcoating it, no way to pre tend he’s better off than he is, no way to avoid what will happen to him. You know that; it’s why you’re scared. Make yourself feel better by researching and understanding resources that are available to him and to you. Realign your thinking to include possibilities you might never have considered, such as the availability of certain therapies in memory care homes. Think about how it would benefit him to see kind faces around him every day in such a place. I’m so sorry for both of you, and good luck.

Yes, he is scared.
And I believe in honesty. There is no answer to this, and honesty is no "the answer"; it is just an answer. A choice.
I would not say a lot, but I would say it honestly, and when my brother got probable early Lewy's Dementia we discussed it ENDLESSLY and he said while he didn't look forward to more losses and it scared him where he was going (said he would so much rather have died, and I replied "But we weren't given that choice", he was STILL so glad to know there was a REASON for what he was seeing/feeling.
I would tell my husband "Hon, you got that stuff we all will get if we live long enough; think of it as brain fog and it will be better some days and worse others. We can talk about it if you want, but we don't have to. Just know we will be together in this as we were in everything".
As to the friend, friend of his or friend of yours is the question? I would allow a friend of mine to know all the details so I could share with them fully. As to friend of his, yes, you should warn him that your hubby isn't the friend he saw last time. That there are some memory (mobility, understanding, comprehension, whatever) issues now. Details are unimportant. That you hope hubby will have memories intact of their shared times (sometimes long term memories are very intact) but that he may not remember, may become disturbed, may even (cry, laugh inappropriately, argue, shout--whatever) or that his reactions may come and go.
I am so sorry for all you are going through. As to what is coming next, no one can know, and that means that you cannot predict what YOU yourself might need to do for his and your own safety. He is ill and failing. You are not. That is reality, and nothing will change it. It is now how best to live with it.
Try not to get out that crystal ball. It isn't reliable. And remember that killing yourself will leave your husband with no support at all. You cannot know what in future you will have to do. And you may be surprised that your hubby would thrive in care.
Again, if he is capable or seems to want to, discuss things. My brother would describe his hallucinations in huge detail, and what would bring them on. He would see things differently as we sat and chatted and while I saw a transit vehicle with black wheels and white fenders he saw a sort of Diego Rivera painting of a Latinex nature with white outfit, calla lillies, a flower market. As Olvier Sacks said, they HAVE A WORLD, it just isn't our world.
Get help and resources. The future is unpredictable for us ALL; we just don't know it, and you now most certainly do. My heart out to you; I hope the visit goes well and I hope you update us.

I'd definitely warn the old friend beforehand or he'll be shocked when he comes by to visit. Also inform him your dh is unaware of his diagnosis so please don't mention it.

FT dementia can be WAY too much to handle at home, in time, so I'd also ditch your "vow" to never place him. Never is a long time and things can go very downhill from here, unfortunately. Leave all your options open.

Learn all you can about dementia so you'll know what to expect and also how to talk to your husband when the time is right. He deserves to know that his memory is slipping and that it's not his fault, that there's disease at play.

Here is a link to an FTD chat group support forum you can join:

https://www.ftdsupportforum.com/

And another online resource:

https://www.theaftd.org/living-with-ftd/support-for-people-with-ftd/

I'm sorry you're dealing with such a diagnosis and wish you and dh the best of luck.

I've never understood why some doctors think it's acceptable to hide a diagnosis from their patient, it's such and old fashioned, paternalistic attitude. I agree that when someone has been told but forgets it isn't helpful to have to keep repeating and possibly traumatizing them over and over, but I feel strongly the initial conversation is absolutely worth having so that your husband has the opportunity to work through his feelings and prepare for his future.

As for the old friend - it's up to you how much you choose to share, you know them better than we do.