Successful hybridization typically requires a lot of genetic compatibility. Sunflowers and cosmos are only distantly genetically related so may not be a very easy thing to accomplish. FWIW, natural cross genus hybrids are quite rare. Might be possible in a lab and with test tubes but I doubt you or your friend can do it just by planting the flowers together and cross pollinating.

Hi whyfrythat,

" But, they have become incestuous according to her so I was thinking
about giving her a flower close enough that she could cross her
sunflowers with them. "

I am going to assume that by "incestuous" your friend means that her crosses are between very similar specimens and the crosses would be more interesting if they were between significantly different specimens. It may be that all that is needed are a greater variety of sunflowers for her to work with. There is a lot to choose from among the sunflowers offered at this link.

Sunflowers at Hazzard's Seeds

Or, your friend might want to switch to zinnias. Zinnias won't cross with sunflowers, but there is a great variety of zinnias that can be crossed and re-crossed with each other. Growing and crossing zinnias is one of my hobbies as an amateur zinnia breeder.

Zinnias have more colors than sunflowers, and more flowerforms. Zinnias can't compete with sunflowers for tall tree-like plants or huge blooms, though. Six feet is about the maximum height for zinnias, with four feet being more common. So you don't need a ladder to work with zinnias.

And zinnia blooms rarely exceed 6 inches across, with 5 inches being typical for "giant flowered" zinnias. Zinnias have a variety of plant habits. One tiny variety (Thumbelina) puts out its first bloom when the plant is only 3 inches high and eventually forms a little 6-inch bush.

No worries about zinnias becoming "incestuous". They do have a lot of variety.

ZM (not associated with any product or vendor mentioned or linked)