Student supervision when student loses motivation to work I am a professor at an academic institution in applied mathematics. Last year around summer, a Masters student in my research group wanted to switch from his supervisor towards my research interests as he became more interested in my line of work. He displayed enthusiasm during my classes with advanced, interesting questions and by coming to my office hours before mentioning that he would like to switch.

After having an extensive discussion with his supervisor, I decided to take on the student's supervision responsibilities. My student worked well on his research for the first two to three months. Given his level of enthusiasm, I expected him to continue on for a PhD with me. So, I was quite surprised when he mentioned that he wanted to end with just a Masters and not continue on to a PhD. He mentioned that he did not know what he would like to do after his Masters, and that he would just like to focus on writing a good Masters thesis for now.

Since then, I had the impression that he has not been putting in enough work every week. His work has been alright and okay and sometimes mediocre. I had the impression that he might have been working on somethings totally different which is not related to my line of work.

Wondering about his interests to guide him in the right direction, I plan to ask him about his future plans and to ask him to just write an okay thesis and put time in to work toward his future goals, be it a job or further studies. Is this the right thing to do as a supervisor? Is the student being ethical here?
Yes, I think you're doing the right thing.

One of my service assignments is undergraduate advising in computer science at a selective school.  I think my experience with undergrads may inform on what could be happening with your grad student.

All the students I encounter are smart and they're all capable of getting an A in anything if they only took one course, had someone else cooking their meals, doing their laundry and cleaning their bathroom and they actually cared about the subject.

When students don't do well, it's almost always one of two reasons.  The most common is that they don't have the time, usually because they're taking too heavy a load, but sometimes due to medical, family, relationship, work or other reasons.

The second reason is they don't like what they're doing.  Sometimes it was a terrible course delivered by an instructor who did a terrible job.  Sometimes it's on us, but those are rare; academia doesn't run completely open loop.  Those student evals actually do matter.

More often, they've been pushed into CS, often by parents, or they've pushed themselves into it.  And they hate it.  They feel cornered and they want out, but see no way out except to plug ahead somehow and get it over with.  An example might be a student who desperately wants to be an actor but is facing threats from his father that any financial support will end if he switches to a drama major.  Another may have realized the work is different than she expected and not what she wants to do with her life but believes she's sunk too much into the degree to give up now no matter how much she hates it.

A social worker friend of mine once remarked, "People don't make changes in their lives until the pain of staying where they are is greater than the pain of changing."  And in the meantime, they struggle and they don't do well.

I'm suspicious from your description that your student has come to a realization that this just isn't what he wants to do with his life.

When I encounter students that seem to be struggling in this way (which is pretty often with UGs), it's often because they had a plan but it's not working and they don't have a new one.  Asking what their plan is when they don't have one isn't as helpful as encouraging them to think about what makes them happy and what they want from life.  What I've found is helpful is to ask them to watch Steve Jobs's 2005 commencement address at Stanford and discuss with them some advice he gives:  To do great work, you must love what you do.  Don't settle.  Keep looking until you find what you love.  You cannot live someone else's life.