Basically, you have to engage in some quality form of RP. Do not just shoot them in the face. If you say, 'Put your hands up' and they run off, yes you can indeed shoot and kill them. If you say put your hands up and instantly shoot, that is RDM, give them time to react If they run or draw a gun you may shoot!
3B) RDM - Random Death Match Definition:
Shooting at someone without engaging in any form of quality role play (eg. Giving enough time for them to comply with your order) is considered RDM. (Punishment is a ban)
Yes if you warn them adequately you can kill them if they try to run away.
'excuse me can you stop right there and put your hands up' *runs* 'sir stop right there! You have 10 seconds!'
*bang bang bang* silly person trying to run dies.
Your friend can shoot them IF they are aware that they could die from robbing you, so you can say 'I have a friend with a gun pointing at you, let me go or die within the next 15 seconds'
OR
He has to make himself know 'stop robbing him and surrender! I want to see hand in the air or you in that car driving away!
If you are robbing someone and they try to make a break for it, you can absolutely warn them to stop. I'm not 100% on you being able to just open fire on them, depending on what sort of initiation you've given them when you started robbing them.
If you are being robbed, your team mates MUST initiate on the robbers before they can shoot them. Assuming the robbers havent restrained you and taken away your radio - you could tell your robbers that if they don't release you, or any harm comes to you, your teammate will open fire on them.
For more concrete advice, join TeamSpeak and jump into the 'waiting for help' channel and speak to a mentor - they know it inside-out and will be able to help you best
Your friend can shoot them IF they are aware that they could die from robbing you, so you can say 'I have a friend with a gun pointing at you, let me go or die within the next 15 seconds'
OR
He has to make himself know 'stop robbing him and surrender! I want to see hand in the air or you in that car driving away!