Strategies for dealing/interacting with a colleague who have suffered from academic burnout
It is a tricky situation, and there is no 'guidebook' way of helping your colleague or how to deal with their recovery - I can only offer an account of when I burnt out.

When I burnt out, my colleagues - both work and academic, rallied around and helped me get back on my feet.  The way they did it was to be there and to simply listen - this was the key thing that helped me rise up.  They also kept things as normal as possible and understood the times when I needed to vent - which over time, became less and less.

They also gave me my space - like an open invitation for me to join in the conversations or activities when I felt I could.

Having also been in the situation from the other side, listening and giving time and space did help.

One more thing, you have to take care of your own well being as well - you are doing the right thing, by being there, it is not an easy road - but believe me, you are making a difference.