Collection agencies affect?

My sister had a negative balance on paypal and had to pay them back which she did and had one final payment to go to which she found out they passed onto a collecting agency, they didn't even tell her about the 45 day rule and she was paying her debt really quickly. Anyway she paid the collection agency when they requested but would it affect her credit score she's so young and I didn't know this was going on I would've paid everything but she hides serious things. Does she need a written letter from them as proof? I don't want her o have a bad credit score because she is so smart with her money. Btw she used a debit card if that matters.