Ask them the exact date and check that 2 weeks (or the full notice period to leave as agreed on initially) are remaining before all rent money prepaid is fully used. Ask them for their forwarding address so you can send on any mail that arrives for them. Ask them how they will get to their new accommodation and assist them if they need help. This does not mean that you have to transport them but they may need assistance in calling a taxi or getting to the train or bus. There is no need to feel that they do not like you anymore or that you have done something wrong. Students are young people wanting to experience different situations. You can rest assured that when students move on it is because you have given them the confidence to do so.
Don't forget to email Study Vision that this bedroom will be vacant, so we may arrange another student, if you so wish.