I'll toss in a few thoughts here.

If they're using a radio in the field, it's most likely a handheld 2 meter and/or 70 cm (440 band) radio. Those are by far the most common bands found on handheld (HT) radios.

There are repeaters located all over the country that receive and re-transmit signals on those bands, usually operated automatically, to access them the user needs to know what frequency the repeater uses for it's input and any PL tones that are needed to activate the machine. Those can usually be found in a repeater guide, since there are so many repeaters around, the guides are usually arranged geographically.

Accessing a repeater will only allow the original operator to get their signal out further, it doesn't guarantee that it will be heard. The best thing to do is carry a repeater guide for the area(s) to be travelled in. Ideally one would pre-program their radios with the needed information beforehand.

There are also recognised "calling frequencies" that one can use for emergency traffic, those are also listed in the repeater guides.

Any Ham, note, you don't capitalise the whole word, knows that they are to give priority to any emergency traffic on whatever band they may hear it. Most normal radio traffic/routing/usage rules don't apply in the case of an actual emergency, in other words, anyone may use a Ham radio to summon help in an emergency.

In addition to any local popularly used frequencies, there is a national calling frequency on the 2 meter band, 146.52 which is monitored by a lot of operators, there are wilderness protocols that involve listening to 146.520 for 5 minutes at the top of each hour, this is an attempt to get people to listen for anyone who might be calling for help. There is a similiar frequency on the 70cm band, 446.000.

Any Ham operator should know to bring along some extra batteries, some will bring along wire antennas that can strung up in trees to increase their range.

A neat trick is using a 2 meter handheld to access a mobile radio that is mounted in a vehicle using what's called "cross band repeating" the mobile radio acts as a portable repeater, relaying the relatively weak signal of the handheld and retransmitting thru the usually higher power of the mobile radio. It's not gonna work for everyone cause you've got to have a mobile radio that allows it to be done but it's something to consider for the Ham operator that's going to be traveling a lot in the wild.

As for the actual mechanics of calling for help, find a band that is in use or use one of the calling frequencies and start transmitting a distress signal, I would probably use something like this, "Emergency traffic, emergency traffic, any station please respond... KI6TED (my callsign) calling with emergency traffic." If there's another conversation going on, wait for a brief pause and then call out "Break for emergency traffic...". The person calling doesn't need a callsign in a real emergency but they should be able to tell any responding station the nature of the emergency, their location as best they can and what they need in the way of help.