Critical thinking
I hope l don't get this messed up but l think these two examples will clarify a priori from a posteriori.
A priori
If John is taller than Mary and Mary is taller than Sam, then John is taller than Sam. Since this statement reduces itself to a logical property , If X>Y and Y>Z then X>Z.
A posteriori
John is taller than Mary. In order to make this statement, we need to have some empirical data.
On the point of science, foundational principles of science are a priori but l would put them into the category of metaphysics. Eg the statement, effect comes after the cause is an a priori statement.
An apple is a fruit.
Dogs are animals.
These statements are also a priori but in my opinion these statements should not be classified as scientific as they merely come from the category we assign to them. It is almost a matter of convention. Even if we give them a certain scientific veneer, they will be foundational.
My friend didn't only regard foundational topics as such but statements like " Gravity causes objects to fall towards earth" to also fall under a priori statement.