Marzipan is often used to cover fruit cakes before the fondant layer goes on. It's almond paste mixed in with things.
It isn't commonly used in the US though.
Marzipan isn't a frosting, it's a paste. An almond paste to be specific. Some people cover cakes with it but it's much more expensive than fondant. It's made with ground up almonds and sugar, you can buy it store bought but it comes in small amounts. I use it for figurines like gumpaste animals but they take longer to harden. If you wanted to make an frosting, I would make an almond frosting, you can add a bit of marzipan to a buttercream to get a similar flavour. But otherwise I would add almond extract to the frosting that you are using. I hope this helps. Good luck! 
If you're in the US and getting an email like that, is it possible it's a scam email? Something like the "Happy Marriage Life" scam but different wording? And the scammer doesn't realize that marzipan is not commonly used here?
If it's a legit request, check the price of marzipan before you quote her. And you'll want to do fondant over the marzipan to make it look right. Marzipan tastes good but a little bitty tube at the grocery store is about seven dollars. Enough to cover a cake would get pretty pricey.
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