For now, it's okay to give it to them if it facilitates the lesson. It's completely up to you though.
We're working on a chat system that will allow you to ask the student to contact you through our systems, and once that's in place, our only strict rule will be to not give out your contact info.
If you remember from your application agreement, giving out your contact info to "poach" a student that you met through HSL is one of the only things that can get you kicked off the site.