Haha! Ask a hairdresser. Not a good idea. But funny. Hair dressers are not sperm specialists as default. And why can't you call them yourself?

But my wife don't like cum on her hair because it's hard to get totally washed off her hair. Small dose of it will dry on the hair. But then it will be quite easy to take off. We have done hairjobs many times and we haven't seen any marks of the cum making some damage for the hair. She has healthy and long hair.

And I'm sure different cums and are different to each other. And same goes for different hairs. So I don't know how your cum affects her hair.

And if she doesn't like to do that, it is of course her right to decline. But if it's not done many times a week then I think it won't harm her hair. That's my non-professional answer.