I have been having this same issue. My wife is very unhappy about this problem of my calendar events triggering alerts on her iPhone. So I've been trying to figure out what is going on.
First about my setup:
* I use Google Calendar as the go between for syncing events between iCal on our Mac computers and in iOS Calendar app on our iOS devices.
* We both use Mac OS X Lion on our Macs.
* We each have our own Apple ID accounts.
* We both have iPhone 4S and iPads (hers is 1, mine is 2) and all are on iOS 5.
* Using Tomasz G. suggestion above I verified I had done that step.
Regarding Tomasz's post, note that when he says to right-click on the shared calendar, this is after first left-clicking on the Calendars button in the upper left corner of iCal window, which drops down a list of calendars. Right-click on the CalDav shared Google calendar there... so for my iCal, I right-clicked in the drop down, on my wife's CalDav shared calendar. On her Mac in iCal I clicked on my shared calendar in her drop-down Calendar list. As he stated, right-clicking gives a menu which includes Get Info.
While checking calendar settings, you might also right-click on your own Google calendar in that drop down, do Get Info and verify that the checkbox for Ignore All Alerts is NOT checked for your own calendar, so that you do get your own alerts. We just don't want to get each other's alerts. So in my iCal, right-clicking on her shared calendar, I checked the box to Ignore All Alerts and the same for my shared calendar in her iCal.
Here is what I've learned from my testing:
* Events set up on iOS devices (my iPhone and both our iPads) with an alert time did NOT fire an alert on each other's iOS devices or Macs, just on our own devices and Mac. Perfect!
* Events set up in iCal on my Mac with an alert time DID result in an alert firing on my Mac and on ALL iOS devices! Wrong behavior! Apple...Google... this is a BUG! IMHO. Please fix this! My wife works nights and she does NOT appreciate getting my daytime alerts on her phone!!
NOTE: To do this testing I had to make sure the event was first showing up on all calendars... i.e. open and refresh all calendars on all devices and Macs PRIOR to the alert trigger time to make sure each device and Mac is aware of the event.