Do you have the Eurail Pass ? It shouldn't cost 300€ but maybe only sleepers are left. Let me have a look, I'll edit down below.
EDIT : seats and sleepers are left while couchettes are sold out. The former cost 14.90€ but you won't be able to sleep much. If you're not on a tight budget I'd recommend the sleeper compartments for a better experience : 104.90€ shared with 1-2 people of the same gender (300€ for a private compartment).
Book through the company whenever possible : nightjet.com -> add Eurail as a discount.
Let me know if it doesn't work.
I am broke so you just saved a girls life! I appreciate you and I hope you have an amazing day!!!!!
Glad it worked!
Hope your friend enjoys the surprise :)