Well, many years ago, I used to work with what used to be called educationally sub normal (dunno what the PC term is these days) people, who were ignorant of some of the basic needs and knowledge for us to get thru life, and they were happy all the time - no worries, nothing.
I think the more you are aware of, be it personally or generally, the more pressure you have.
whilst dementia sould not be joked about it does have its upsides. My boss' mother suffered from it and one day her grandson, whom she'd never met arrived from Australia. During his stay the boss took him 3 or 4 times to see his gran and each time she was over the moon 'cos she thought it was the first time they'd met....
In response to your second question about 'your mate's other half'' playing away from home - yeh, I would tell.
I would tell my mate that her partner's change of behaviour might possibly be because he is shagging around?
Married people don't just change their behaviour and she can make what she wants of it. Ignorance is her choice?
Im not a fan of ignorance is bliss, I prefer to know the facts, however difficult they may be for me to accept, but it depends on your personality type, some people cannot accept hearing harsh realities and so tend to wear blinkers
But to add to that - If she didnt raise the issue about her partners changes, then I wouldn't say anything as she may know herself and not want to raise the issue? Whatever her reasons.