How does she get the messages? Whenever I travel, I purchase an international SIM card or a foreign SIM in the country. You get a new number so basically, you put the SIM in and use that number. Unless El Salvador has her phone carrier, it's impossible that she can get a message to her phone.

Unless she is using an app? Textnow?

how close are you? when I travel a barely stay in contact with anyone except my parents because my mom will worry to death but I have communicated with my folks from all over the world, even remote mountain villages in Nepal or in the Amazon forest. sometimes in areas with weak wifi video-calls don't work but texting always works. does the icon say the messages were read or just received? usually there are 3 stages : 1:sent but not received, 2: received but not seen, 3:seen. maybe if you are worried you can contact family of hers who she would definitely be in contact with?. one time I was in Peru and forgot to write to a friend of mine to tell him I was ok and when I did he was freaking out.

She can send and receive when she has wifi connection or within cellphone coverage.