You can get pen ink out of jeans with 90% isopropyl alcohol.
Take a clean white towel and place the clean white towel inside the jeans under the stain so that the pen ink does not soak through to the other part of the jeans.
The towel also helps catch the pen ink as it comes out of the jeans.
Then take 90% isopropyl alcohol and pour it over the pen ink stain and allow the 90% isopropyl alcohol to sit for a minute or two.
Then use a paper towel or something to blot the pen ink stain and the pen ink stain should come out.
You can also spray the pen ink stain with some hair spray and then blot the pen ink stain as well.
Then wash the jeans in the washing machine to further and entirely remove the pen ink stain.