I want to call the police on someone stealing sanitary products
Why do you think that it's necessary to do that?
I think anyone who saw shoplifting would do the same, right?
I would get frustrated and would report someone if I found that they are stealing as well, so I think that it's just appropriate for you to call the police even though the items that are missing are very low in value. I would make sure that you have the necessary proof about the items that are missing, so that you can make sure that the case would not be dismissed so easily by the police.
What if it's my best friend?
If it's your best friend then you need to treat them like you treat anyone else. If they stole then it's the same crime and should still be reported.