What about this one from Martha Stewart?

https://www.marthastewart.com/1527194/rhubarb-crumb-cake

And there's a buckle recipe on the site as well:

https://www.marthastewart.com/898912/rhubarb-buckle

This cake could have a struesel added. I've eaten it and made it and it's good and simple --

Martha

RHUBARB CAKE

1 package yellow cake mix (regular size)
4 cups chopped fresh or frozen rhubarb
1 cup sugar
1 cup heavy whipping cream
Whipped cream and fresh mint, optional

Directions
1. Preheat oven to 350°. Prepare cake batter according to package directions. Pour into a greased 13x9-in. baking dish. Sprinkle with rhubarb and sugar. Slowly pour cream over top.
2. Bake until golden brown, 40-45 minutes. Cool on a wire rack for 15 minutes before serving. Garnish with whipped cream and mint if desired. Refrigerate leftovers.

Here are what i have in my file

#2 ( that's what it's called!)

Ingredients:
1/2 c Margarine
1 1/2 c Sugar, brown
1 Egg; beaten
1 ts Vanilla
1/8 ts Salt
1 c Buttermilk
2 c Flour
1 ts Baking soda
3 c Rhubarb; finely sliced
1/2 c Nuts; chopped
1/3 c Sugar, brown
1/3 c Sugar
1/2 ts Cinnamon

Procedure:

Slice the rhubarb into 1/4" slices. Cream the margarine and 1¼ cups brown sugar; add the egg, vanilla and salt; mix well. Add the buttermilk, flour and soda; mix well. Stir in the rhubarb. Pour the batter into a well-greased "x13" cake pan. Mix together 1/2 cup nuts, 1/3 cup brown sugar, 1/3 cup white sugar and 1/2 teaspoon cinnamon; distribute this mixture evenly over the unbaked cake batter. Bake for about 45 minutes in a 350 degree oven.

This is called a pudding cake..Source: Jean Kressy, Cooking Light
MAY 2004

1 cup all-purpose flour
1-1/4 teaspoons baking powder
1/8 teaspoon salt
5 tablespoons unsalted butter, softened
1-1/3 cups granulated sugar, divided
1 teaspoon vanilla extract
1/4 teaspoon ground cinnamon
1 large egg
1/2 cup 1% low-fat milk
4 cups (1-inch-thick) slices rhubarb (about 1 pound)
Cooking spray
1 teaspoon powdered sugar

Preheat oven to 350°.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour, baking powder, and salt, stirring well with a whisk. Place butter in a large bowl, and beat with a mixer at medium speed until smooth. Add 2/3 cup granulated sugar; beat until well blended. Add vanilla, cinnamon, and egg, beating well. Beating at low speed, add flour mixture and milk alternately to sugar mixture, beginning and ending with flour mixture; beat just until smooth.
Combine rhubarb and remaining 2/3 cup granulated sugar in an 8-inch square baking dish coated with cooking spray. Spoon batter over rhubarb mixture. Bake at 350° for 45 minutes or until a wooden pick inserted in center comes out clean. Sprinkle with powdered sugar.

And another>

Rich Ranch's Five Generation Rhubarb Cake

(This recipe came to Montana in 1865!)

Ingredients:
1-1/2 cups brown sugar
1/2 cup butter
1 egg
1 teaspoon vanilla
1 teaspoon salt
1 teaspoon baking soda
2 cups flour
1 cup sour milk
2 cups raw rhubarb (finely chopped)
1 cup walnuts (chopped)
Topping:
3 Tablespoons butter
3/4 cup brown sugar
3 Tablespoons cinnamon
2 Tablespoons flour
Cream together brown sugar and butter. Add egg and vanilla, then add salt, baking soda and flour. Mix in sour milk, rhubarb, and walnuts. Pour into a 9 x 13-inch greased cake pan. Mix together topping ingredients and sprinkle over cake mixture. Bake at 350 degrees for 45 minutes. Tastes great served with vanilla ice cream or whipped topping.

This is a keeper. Very easy and delicious.

https://recipes.sainsburys.co.uk/recipes/rhubarb-cakes

Thanks, everyone, for taking the time to post. I've looked at all the suggested recipes and some sound very close to what she made. Now I just need to do some baking experiments! :)