How often does your friend test his/her blood sugar?
Most doctor's prescribe a long acting insulin (such as Lantus) to be taken daily and then also a shorter acting insulin to be used on a sliding scale. The person tests their blood sugar before each meal and at bed time and compares the result to the scale prescribed by the doctor and inject the appropriate amount of insulin (if indicated). It is a good idea to record these levels in a journal so it can be shared with the doctor.
The idea is to keep the blood sugar at a steady, consistent level. If your friend's sugar is not consistent, he or she needs to see their health care provider for a thorough check up. There is a blood test (HgbA1C) which will show his/her blood sugar levels for the past 3 months and will indicate to the doctor how well controlled your friend's diabetes is.
You're a good friend to be concerned about your friends well being. If "N" is Humulin N, there is no "standard" Humulin N dose that will work for all people or even for the same person in all situations. This applies to other insulin types too. If your friends at home test results show low glucose levels "several times per week" it is time to have his/her insulin dosages re-evaluated by their physician.