I would like to make this dishcloth into a blanket for a friend who loves sunflowers. What would be the best way to convert the pattern?

You can make the center bigger by working the increase row till you have made as many bobble as you want.

The petals you cast on more sts and do more shaping.

That is so cuuuuute! Yes, it would make a great blanket. I would suggest a different yarn, if you can get the right colors. I'd recommend Light & Lofty. Exactly the same yarn is also called Baby Clouds, but that line only comes in baby colors.

Light & Lofty or Baby Clouds both work up fast, hold their shape and color beautifully, go through the washer and dryer like a dream, and weigh next to nothing -- even in blanket size!

It also works with a bigger needle. Read the package for the correct sizes.

How about making individual squares (just follow the dishcloth pattern as is, except using a soft yarn rather than the normal cotton yarn) and sew them together.