That's definitely tricky, because every family works differently and their financial situation varies! From what I've picked up in conversations, the range for 15-year-olds is pretty wide - some get 20-30 euros a month, others get way more. A lot depends on whether she does chores around the house (and whether those are paid) and whether she's able to pick up a little side job. If your daughter's supposed to buy her own clothes and cosmetics, she'll probably need a bit more than if it's just for leisure spending.
The main thing is really that you two agree on something - not what her friends are getting, but what works for your family! Maybe it makes sense to think through it together: What do you pay for anyway (phone, insurance, etc.), and what should she cover from her own pocket money? If cosmetics and fashion are really supposed to come from her own money, then of course she needs more than if she just wants to go to the movies with friends. Some parents also gradually increase pocket money as kids get older, instead of jumping up in bigger amounts all at once!
One more tip: If she really wants more, she could always help out actively - not just standard chores, but extra things that get paid. That also teaches her that more money comes from more effort, and it takes the pressure off the whole "my friends get more" comparison.