Welcome to cake central. It is sometimes called a poke cake.
you bake a regular cake 9x13, let it cool, then using chopstick or similar object, poke holes over the cake. then pour cooled, jello over the top.
It sinks into the holes. Chill it well. Cover it with whipped topping, and it is ready to eat or chill some more.
I've never heard this, so I can be of no help, but it sounds like a great idea. Sounds like kids would love it too!!
I know it as a jello cake. You just bake a cake like you normally would. When it's done, mix up a box of jello like the directions say. You don't have to chill it. Take a toothpick, fork, or whatever and poke holes all over the cake. Pour the jello over it and then pop it in the fridge til it's cooled. I am used to using the small box of jello for a typical 1 mix size cake. It's really yummy served with fresh whipped cream!
We make it all the time it's a summer time tradition 
We normally take a 1/4 sheet cake and once out of the oven still in the pan I poke holes in it and pour prepared jell-o over the top, filling the holes (many many holes ) pop it in the fridge for a few hours until chilled just as you would if the jell-o was in a bowl, and then we top it with cool whip and fresh fruit to compliment the jell-o. We normally use strawberry jell-o and top with sliced strawberries 
ummmm buttercream n sweet stuff... Sorry but Blech blech blech.... Just my personal opinion hun, I would try to steer her towards a less sweet option on the frosting if you could. Just tell her that the cake would be too delicate to use that heavy of a frosting (same way with 3-milks cakes). That it would probably squish and be just ugly lookin (even though you have wonderful talent). The cake would just collapse, at least that's the experience I have had when people want 3-milks cakes and wants american buttercream on em. I just flat out tell that that isn't an option. You may try whipped cream buttercream or italian meringue buttercream or even if you can get it use the non-dairy whipped icings that bakeries use.
These cakes are so yummy! We always have them in the summer. I have to agree...do not use buttercream. I've tried it (only because it was insisted on by customer) and it was a mess. The cake is so wet and moist, it just kept tearing. And the combination did not taste good at all. We usually just use whipped topping. Good luck.
Jell-o has a new Margarita and Strawberry Daquiri jello I plan to try in a cake!
I went to my local grocery store bakery and asked about roses... I can't make them and they sell them to me for really cheap lol... Anyway she was willing to sell me a small container of thier whipped icing that they use for a similar cake. It's very light and fluffy it's not very good for doing detailed decorations but it's alright for borders I think. Maybe you could call them and see about getting maybe 3 cups of it or so... that's about how much is in a batch of buttercream
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