How much can you help with someone else's thesis (without getting credit)?

I have been working in the same lab for two years now (as an undergrad) and have just begun as a masters student. Today in a meeting with my supervisor, he wants me to collaborate with another one of his masters students in order for her to be able to graduate in time (she is nearing the 3 year limit imposed by the university). He said the word “collaborate” and so he expects me to implement the algorithm (its a simulation type thesis so you code an algorithm, simulate it and then analyze the results and compare with other algorithms).

However, I’m already helping the other student out considerably with setting up the simulation scenario and by the looks of it I will also have to help with analyzing the results (she always comes to me to help her figure out what her results mean and what she has to do next). To me, this seems to be one sided helping since I can’t see what I get out of this “collaboration”. Furthermore, I’m afraid of doing a lot of work and seeing her get a masters for it. My supervisor’s justification was that my thesis will be an extension of hers so helping her finish is what I want but personally, to me it just seems like I’m on the hook for two masters thesis and I’ll have to make her graduate before I can even work on my own thesis.

So the question is, are my beliefs/fears correct or is this common practice in labs? Further, how much help can you ethically give for someone else to complete their thesis if you are just a younger student starting your own masters (i.e. where do you draw the line at help before it becomes your own work)? And, if my beliefs are correct, how should I procede? Any suggestions/advice would be much appreciated.