"I will constantly wonder if she meant what she said"
If she is I'll enough to be sectioned, then she has had an "episode", this involves irrational or delusional thinking, so whilst having this "episode", she isn't responsible or means what she says/ how she behaves.
"Now i feel sorry for her partner sitting next door wondering what on earth is going on"
Why do you assume that? He may have a good understanding if her condition and has been through this before.
Her daughter was right to tell you to go home.
I wouldn't go into her house again, just in her request, that will protect you and her, in the future, if she is behaving as though things aren't going well.
Did you not see the recent adverts that put people in everyday situations, work, social etc, after being sectioned or treated and how relieved they were that those around them behaved normally and asked how they were?
That us what you would do after a neighbour had suffered a fit or heart attack infront if you and you saw them again.
Drop a card through the door, for her DD or DP to take in when she is feeling better. A thinking if you card, or a small gift, if that's what you would if done if a neighbour was hospitalised after being knocked down I front if you.
This is what removes the stigma and makes MH as any other medical problem.