Where I live, it's quite common for MSc students to seek help from "professionals" in this field. What they do varies from simple peer-review-like service and in the worst case they provide a full thesis with tuition i.e. spoon feeding.
It's amazing how many people teachers and teaching assistants are (unofficially) accused of getting this type of degrees. But this accusation in most of the time actually holds due to obvious lack of fundamental comprehension of the topics they are specialized in.
- Peer review is quite common and a prerequisite for papers to get published in a respected journal.
- It's typical for a professor to have one or more teaching assistant to help him doing some part of the work for example some CS professors delegate the proof of concepts implementation to a teaching assistant.
Besides the two examples above what else is allowed and considered an acceptable academic activity by a peer:
- Translation for thesis written in another language to English which is the required thesis language in that university.
- Writing a proof of concept off the record i.e. without citing the PoC creator in the thesis officially.
- Writing the (Related Work / Literature Review) given the student claims full knowledge of the papers but having no time for writing it.
- Does the above changes when it's done as a service i.e. paid, instead free peer assistant?
Note 1: I do have my own view for this but I would like to hear yours. Also I will mention my view later in a answer for this question.
Note 2: I'm concerned with the STEM fields if that makes a difference.