My friend has dementia. Her live-in boyfriend of 9 years is putting her in a memory care facility, which she doesn't agree to or know about.

I think putting someone in any kind of facility and disappearing for 3 months is not only cruel, unhealthy and negative, but it's irresponsible.  I find it difficult to believe any administration of any facility would recommend this.   I really doubt she'll become "better situated", but I would bet that she becomes more confused, frightened, and perhaps even rebellious.

I have some other unrelated concerns though, arising from his legal ability to place her.   This man may be a live-in BF, but has your friend created any powers of attorney for financial and medical actions?   Very importantly, WHO is paying for her stay?

I've often heard people here on the forum say they were given advice to stay away for a few weeks (not months), it's not something I personally would have ever agreed to do.

Yes but not 3 months! More like 2-3 WEEKS