Can I start wearing helmet 42 days after a FUE hair transplant?

You can resume your normal activity well after a month after surgery.  If you feel there is an issue it is best to follow up with your doctor.

Sure. After a month past your procedure you can essentially go back to your normal life. Your grafts are as strong as regular hair now so you don't need to worry about damaging them any more than you'd worry about damaging your original hair. A helmet would be perfectly fine.

At 42 days you are fine to wear a helmet and get back to your normal washing routine. Make sure to keep the helmet clean.