You have a friend in Russia who is young, female, and living with her parents. She is a student and currently between semesters. You have never met in person and you want to invite her to Germany and that requires that she obtain a Schengen short-term visa. I assume that she has no prior history of travel from which her performance can be reckoned. She is having anxiety that her circumstances may cause her application to be refused.
Is it likely that she won't get the visa?
The outlook is poor. While everyone who wants to will begin international travel at some point, you gave no indication why a visit to Germany is appropriate for her at this particular point in time. It means her premise is weak and possibly untenable. Moreover, she has things which elevate the risk in issuing a visa to her.
If so is there anything we can do about that, like proving she has a return ticket / reservation?
Reservations (or a credible itinerary) are a requirement for Schengen visas, so having these things simply qualifies her to submit an application. They are easy to obtain and do not prove intent.
For the rest of your question, sometimes it is hard for a young person to acquire quality evidence because they are just starting out in life and have not built up the social and economic standing needed to be successful. If all she can present is low quality evidence, like school enrolment, your best course of action is to advise her to wait and propose an alternative.
For alternatives, you can consider meeting her in Russia. September in Moscow affords lots of great things to see and do before winter sets in. Another popular alternative is to meet in Turkey. The travertine terrace formations at Pamukkale, for example, is a great venue in the autumn. Meeting in person has the added benefit that it provides a premise for future applications: to maintain personal relationships. It's a great premise when it's properly established.
For your implicit question: is her anxiety well-founded? Yes, it's likely that someone in the circumstances you have described will have difficulty succeeding. Overall, the lack of a viable premise will likely be the show-stopper. The refusal reason associated with this is "Justification for the purpose and conditions of the intended stay was not provided" and sometimes they will throw in the generic reason: "Your intention to leave the territory of the Member States before the expiry of the visa applied for could not be ascertained." for icing on the cake.