Irregular periods are caused by a number of things. Acne is not one of them. Acne can be a symptom associated with irregular periods in that both may be caused by a hormone imbalance. Irregular periods at 17 is fairly common because the teens hormone production hasn't become level.
There are many OTC treatments for Acne. I went to a dermatologist who prescribed hydrocortisone cream before it was sold OTC. That cleared up my acne. I had irregular periods any time I wasn't taking birth control pills. Having irregular periods can be "normal" tho very inconvenient. Mine were also painful.
As an adult, I was eventually diagnosed with endometriosis. I was treated first with birth control pills which did make my periods regular and less painful. Eventually I had a couple of surgeries to correct the endometriosis. After surgery my periods were shorter and less painful but still irregular.
I think that endometriosis still only has two available treatments: hormones and surgery.
Irregular periods can be caused by several other conditions. An ob/gyn may or many not be able to diagnose the cause and then may or may not be able to successfully treat it.
Your post sounds like the mother and teen want to correct the acne because they think that if the acne is treated the periods will become regular. Hormone treatment may correct both conditions but then again it may not.
If her acne is severe she will most likely need prescription medication to bring it under control. However, she may be able to reduce the acne with twice daily cleansing of her skin and an over the counter treatment. For some, washing their face twice daily using appropriate cleaning products along with a having regular nutritious diet will reduce the breakouts. It's important to not touch the face or pick at or squeeze pustules. There are several good sources to learn how to care for ones face as a teen. If her acne is severe and home and otc remedies are not effective I recommend seeing a dermatologist to prevent scarring.
So, what I would do is work at finding an effective way to treat the acne using home and/or otc products along with good hygiene and a nutritious diet.
Based on my own experience with my own and other's irregular periods I would wait until the teen is older unless there are other symptoms that may indicate something serious is going on. If she has pain she can take 800 mg of ibuprofen starting at the first indication that her period will start soon. Take the ibuprofen even if there is no pain. Ibuprofen influences our own "chemical" responses to the period and may prevent pain to start. Ibuprofen is a pain killer but it also affects other processes in our body. I don't remember enough to explain it.
The tea sounds terrible. Do you know the credential of the "chinese herb doctor?" Anyone can say that they're a "chinese herb doctor" without having any training or experience. I would recommend seeing a naturopath. They receive years of training and are licensed. It is quite possible that the naturopath could recommend natural ingredients such as teas which will be beneficial.