If she has decided already to run it, she can start at one pill/day and increase one pill /day until she can't handle the buzz anymore. Then she can back off one on the following day and stick w/ that. Is she planning a 2 weeks on / 2 weeks off cycle?
The body develops a tolerance for clen so she may find that she is feeling it less towards the end of the two weeks while holding the dosage the same. Then she needs to go off for 2 weeks and let the receptors recover so they are sensitive to the clen again. On days when she does more than 1 pill/day, she might want to split the dosage up between say morning and noon dosages. The issue is that if you take it too late in the day, you can't sleep. That just screws up your body's natural recovery period.
The reason you can't really guess the dosage is that people react differently to it - particularly with regard to how they handle the jitters. For me, I end up riding the edge of an anxiety attack with daily headaches for 2 weeks at a time if I try to do it. I personally hate the stuff. So just start slow & build up to where you can handle it.
I'd also throw out that clen isn't exactly a miracle drug if she's looking for quick fat loss - it is slightly better than your typical thermogenic at fat burning. Other options (or things to include with the clen) are to look at her diet & maybe tweak it a little bit for improved fat loss.