Personally, I think it is impolite to host a party and then expect your guests to pay. I realize people do that, I've even been to a dinner party where the "dutch" part was announced when the check was delivered.
If she cannot afford to pay for all of them, perhaps she could scale back her plans and just go somewhere for dessert. Or maybe if she has another friend who has a birthday close to hers they could team up and have a joint birthday dinner. Or what if she made dinner for her friends and invited them over? Encourage her to be creative and think of a fun and "within-budget" way she can host the party.
I don't think there's anything wrong with you helping her with "the budget"... but I also think its fine not to.