The spell you need is something like
bob sidechat "Get me outta here!"
which goes in the activation field of a trigger (or in a script). As long as your buddy called bob is still alive that text will appear at the side of the screen. sidechat is visible to your side: I think there are groupchat and globalchat options as well. The text has different colours. Suspect sideradio is the same as sidechat, but not absolutely sure.
Check out the [un]official command ref for more deets. If you know (or suspect) a keyword its always worth checking there ... I always look at both because then you get two different takes on the same thing which makes it easier for my simple brain to understand.
So you could have trigger over the ambush area set to west detected by east (or whatever) with that line in the activation field. If bob had cashed in his chips the message wouldn't get sent so you could have another trigger W det by E with this in the condtion line
this and (not (alive bob))
and this in the activation line
betty sidechat "Ambush! Bob's dead!"
Remember that if you release this mission publicly some poor schmuck like me is going to play it differently so you have to cover all eventualities.
Hope that helps
PS - to make sure you don't get both messages you need to use a little variable.
bettytalk=true goes in init.sqs
bettytalk=false in the activation field of bob's trigger
this and (not (alive bob)) and bettytalk is what actually goes in the condition field of the second trigger.
That way, if bob says his piece then betty will keep his trap shut. But if bob catches too much lead then betty will do the talking.