If your friend went to bed at 1:00am on Friday and woke up at 1:00pm on Friday, she slept for 12 hours.
To see why, you can count the number of hours between 1:00am and 1:00pm:
1:00am to 2:00am = 1 hour
2:00am to 3:00am = 1 hour
...
11:00am to 12:00pm = 1 hour
12:00pm to 1:00pm = 1 hour
The total number of hours between 1:00am and 1:00pm is 12 hours.