Lansbury has a rep for being nice and gracious. I'm sure a 'it's so nice to see you in person; thank you for responding to my fan letter' would have been fine.
She would have smiled and acted gracious.
But I agree with the posters who believe that it's best in this case -- since OP's pal was WORKING -- it was smart to err on the side of saying nothing.
This isn't about approaching celebs, but about respecting a friend's work and LIVELIHOOD.
I work in the press and brought along a girl (friend) to an event where there were sports figures.
Everyone was on the same page that I was bringing a guest -- not a collegaue. It was cleaered in advance with p.r. reps and my boss.
When we were getting name tags, I gave mine and my outlet's at the check in for the event. My friend (when asked how her name tag should read) joked: "Available!"
She laughed. The girl checking us in laughed. And then she gave her name and that's what went on the tag.
(I saw my career go out the window, but I was overly cautious.)
I covered the event without incident and my friend behaved professionally inside.
The next day, though, I was called into my boss' office. Boss wanted to know why I i.d.'d a 'guest' as a 'co-worker.'
I said I didn't and asked what led to that impression; my boss uttered a name and admitted that person wasn't at the event; who knows how it happened.
I stood up for myself and said it sounds like someone who wasn't there is having a conversation with you, who also wasn't there, about something you think happened. But it didn't. We set up in advance that I could bring a guest and that's what I did.
It was all forgotten pretty quickly, but I thought twice about bringing a friend to an event.
All this said, however, saying hello to Ms. Lansbury at the end of the show, on the way out, wouldn't have been the worst thing.
But moments like that have to be timed precisely so there's no awkwardness.