Hand them the deck and tell them to do it for you.
If this is a joke - it comes across as advice, and is cliche magic mal-practice. Even as a joke, it really lacks.
Anyhow - this is a good question that hasn't really been addressed yet.
It is a hard situation - a magician before you has left a good impression about magic, enough that this person wanted to share their experience with you. The audience member doesn't realize that this is the WORST timing, and that it really steps on your moment. It would be like me talking about how much the last table tipped me, or how fun they were...awkward pressure - but that is not their intent. Magic is so powerful, that the thought magic makes them instantly snap back into that feeling of magic that they had...which creates emotion that they want to share - hopefully excitement connected with being fooled.
I have handled it many ways - I was once frustrated when a women be-littled me without realizing it, by saying..."Oh my uncle does this thing with a "Poof" (simulating a fire ball - flash paper) - are you going to make a "poof" - it totally killed the moment, but what killed it more was that at the time...I wasn't sure how to handle it. I said, "Your uncle sounded like he did cool stuff, I don't do anything with a "poof", but I hope to show you something that is even more amazing". Wrong answer - lost the crowd trying to be a nice guy.
I have even tried being a jerk - and we know how that turns out - you lose the crowd again.
So, here is what I found works. Really, I slap myself for not thinking of it earlier. Ask questions. That is it really - ask them how it made them feel, what they liked about it - At this point - you could segway into your effect with a question - let's say you are doing G-Force - you could say, would you consider what he did - EASY magic or HARD magic? Then go into what you mean with your cards, after responding to the answer.
OR
You could just state - Do you consider him a great magician? Yes. GREAT - because I have done magic for X number of years, and have studied magic so well, that often I have to buy the same book twice because the pages fall out. The hardest part of what I do, is approaching a table - most people think it is going to be a horrible experience, because Uncle Milton did a card trick where someone picked a card...he dealt through the deck for 30 minutes, to find the wrong card. Conclusion - magic sucks. However, you saw someone that loved the art of magic, and it left a great impression on you. I intend to do the same, is it okay if I share with you some of my best pieces of magic?
It doesn't hurt to ask the person's name - once I had someone say Ross Bertram, often...it is someone I know from out of province or country. If they are local, you could hook up with them to talk ideas...this puts the spectator into a odd spot - you aren't intimidated by her memory, you want to hear more about it - and perhaps meet the guy that she had a good experience with.
In short - validate her feelings on magic. Don't brush over it. She is sharing. Once she is done living that moment, your kindness will warm you to the crowd, and you can share your crappy magic.
Cheers.