Are the blades tied down?
If you could contrive to have the running down from a previous trip, that would help, where the pilot has shut down but left the machine to help with the plasma bottle or something. Otherwise, it takes about 12-15 seconds to flick the switches and get light off in a 206. If it was an L, he would also have to remember to turn the fuel valve on.
A 500 might be better, since the blades won't be tied down and are low intertia, but that might not be used for medevac. Didn't James Bond start one in the back of a transport plane?
It was in colour, so it must be true.
Dennis Kenyon did a quick start with an Enstrom on a ship in his novel. Took a couple of rounds while it spun up if memory serves.
Sounds like you need a reason for them not to suspect that he is getting away in the helo.
Phil