I dont have alot of experience yet with php, but it seems very doable. The calendars already have private and public events, surely it should be possible to tweak things so you can see another users private events.
If needed you could even log in under their username to look at their events, but that tosses all security concerns right out the window.
Wish I could do it myself, good luck. Ask me in a few weeks and I bet I'd be able to do it.