I agree with a lot of what the other mom's posted and would do all of the following:
1. get the parents on board, if not, do it on your own.
2. have the girl sit on the potty on a regular schedule
and for a longer period of time, like 15 min twice a day always at the same time. She needs this designated time so that her mind and her body can learn to relax on the toilet. Also, with this regular rhythm eventually she will go more often. Over time, try to make this sitting time as relaxing as possible, have her read a book or something else. She needs to relax to let go eventually.
3. give her lots of everything that can help including the prune juice
4. start her on probiotics and enzymes, if the parents agree. They are out there for kids too (i.e. from Kirkman)and with them you are only giving the child something which should be in her gut anyway. If giving her these helps, that is one indication that there might have been something wrong and at the same time you are helping fix it.
5. For starters, put her in a diaper, slid the back open and have her sit on the toilet. That way, she can gradually learn the feeling of pooping on the toilet with the security blanket of feeling the diaper. Give her lots of encouragement once she goes and gradually take it from there, for example after a few times going like this she could maybe only hold the diaper while pooping on the toilet.
Of course you are absolutely right, all this only works if you consistently keep at it. And don't expect wonders, if now she is going less than once a week, I would not expect to see quick results.
Explain to the parents that all of you need to work on this to see if this is just her wanting to feel powerful or if there really might be something in her gut. As long as you haven't gone thru all of this, it will be impossible for a doctor to decide this too.