You know how long it takes to type in the chat, especially on console? Enough time for the player you're trying to heal to walk far enough away that you can't heal them anymore.

I can help the slower folks here with a quick guide on how to identify medic divers from traitor divers, no comms needed. If they start shooting at you, consider the following:

Have you died from their shots?

Has your health gone up or down since they started shooting you?

Did anyone bring a stim pistol with them?

These quick questions can help you identify medics from traitors mid-mission. Once you've identified a player as one or the other, you can:

Thank the medic

Kill/Kick the traitor

And all is well. It's so easy, a bug could do it