Does not paying your credit card in full hurt your score?

Yes.

In fact

best practice is to pay your credit cards BEFORE the total equals 10% of your total credit.

So - for example - if you have a total credit limit of $10,000 in all of your credit cards

then for best credit rating

you should keep your total credit card debt to < $1,000.

That means: for the best credit score in that example

every time your total credit card debt approaches $1,000

you should pay the credit cards off.

So: your "student friend" is actually doing the best thing to keep the credit score as high as possible

but it probably makes a difference of < 50 "credit score" points.

Someone with established credit

and/or a high-paying job

probably does not need to concern themselves with such small amounts of "credit score" points.

As long as payments are on time and in excess of minimum balance due, your credit rating will go up. It shows you NEED them. That's where he shows his value to them as a cash cow. The trick is, once you get that marriage, get rid of credit card debt ASAP.