My friend is 15, and her bicuspid/premolar tooth fell out, does that mean it's her baby tooth falling out , or did her tooth permanently fall out?

Delayed eruption?: It's most likely a primary (baby) molar that came out but your friend would need to be personally examined by a dentist to determine which tooth came out and if it's normal or not. X-rays would also be necessary to determine if this is part of a delayed eruption pattern or something more.

Mpst likely a baby: most likely a deciduous molar, check with a dentist

Possible baby tooth: It may have been a baby tooth, my medical assistant lost a baby tooth at age 38. If concerned, see your dentist for examination.

Examination: Highly unlikely it's a permanent tooth. Probability is that it's a baby tooth. Question is whether there's an underlying permanent tooth to replace it, or perhaps the permanent tooth is congenitally missing. Your friend should see a Dentist immediately for an examination. If there is an eruption problem, or missing tooth, your friend is a candidate for "Posterior Bite Collapse." See Dentist.