i don't think there is any function to target a random friend
maybe create a macro similar to
Code:
/targetfriend yourmain
/targetfriend a
/targetfriend b
/targetfriend c
/targetfriend d
...
/targetfriend z
/cast Chain Heal
it won't be random per say, but depending on the people around you you'll target them alphabetically and it could turn out to look a bit random :-)
(and it will fall back on your main if none other are around)