I'm on the other side of the spectrum (a student who is remotely working on research with a professor for the summer). Perhaps my perspective can help.
Working from home without ever having met the people one is working with on something that they are probably new to, can honestly be quite demotivating and I sympathize with the student. This was something I was pretty worried about as well. Academia SE gave some really nice advice in this regard (see near the end).
Some initiative that my professor took that really helps me:
Expectations and Direction
As stated in the other answers as well, have a meet (preferably on Zoom or Skype) to discuss your expectations of what the student needs to work on over the Summer.
Don't just describe the broad area of the research and a general direction (this is also necessary). Discuss the particular direction of the project and the work that it involves. This gives the student some clarity and not make them feel like they are wandering around aimlessly. The more specific, the better (although I understand it's often difficult to give specifics while doing research, as things can go in unexpected directions. Just try your best).
Just as important is for you to ask the student about their expectations with this project. What skills are they looking to learn? What is their end goal for the project? This will ensure that you are both on the same page and can work together proactively towards that goal (provided it's feasible).
This is really important. Have regular meets on Skype or Zoom, preferably with the camera on, to discuss their progress. This adds to the accountability for the student and it is much easier to ask questions on a (virtual) face-to-face meeting rather than email or Slack.
Socializing with the group
This may not be possible under all circumstances, but if it is possible, it can be so helpful! You might be part of a research group under a Professor, right? Having a weekly group meeting to give updates or a journal club to discuss papers with the entire group can be really beneficial, not just from a research perspective (if two people are working on similar projects, it helps to let ideas flow through a discussion) but also allows the student to socialize with the rest of the group and ask questions more freely. If there are other REU students, it will be nice to meet them. This really helps with morale.
I live on another continent to the rest of them, so for us, there is a nice inter-cultural aspect to it too.
Talk about the big picture
Don't just go about it mechanically, where you give the student a task, they complete it, you review it and so on and so forth. Let the student know where their work stands in the grand scheme of things. Tell them about recent developments in the domain. Encourage them to read papers and think about their relevance to their own work. Again, this helps with the motivation for the student and helps make their work more meaningful.
Seminars and Colloquia
I am not entirely sure, but I assume if the student had been on campus, they would have been allowed to attend seminars that your department may hold? It is probably happening virtually now, so encourage your student to partake in it, even if it is not directly related to their research. All this is again part of making the REU experience more valuable and 'real'.
Things the Student Can (Should?) Do
I was really dreading the possibility that the virtual nature of the REU and a few difficult months prior (mental health wise) may completely demotivate me. I asked this question on Academia SE and they really came through (as usual). I am following the advice, and it's enhancing my research experience in many ways. I encourage you to share their advice with your own student. It essentially boils down to:
- Keep a log of your work.
- Type up notes of everything you learn and do.
- Have regular meets.