I seem to remember a post by "Frogboy" awhile back on alliances that pretty much stated that the AI gets to decide on how to support allies in war with other races. An ally at the outbreak of war will come to your aid if they are not allied with the race that declared war on you.
If you declare war on an alien that the ally is not allied with, they get to choose if they want to come to help, generally I have noted that after you have "kicked the lions share of booty on the aliens fleet." your friends come a running to help you with the clean up (and subsequent military "colonization") of the planets your star troopers haven't had a chance to get to yet.
If the AI is allied with both of you it can and does freaking nothing as far as I can tell, course, on the other paw, I or them normally don't last to long. When war is declared either I am ready to lay the proverbial kat smack down or I get the kat smacked outta me...
W/R
Suralle Straykat
Kat Lord @ Large