It could be a lot of issues, your friend will likely have to go by process of elimination.
PG is a drying agent, so yes,
throat could be getting dry and she may need to try a VG liquid. If she's using a mouthwash with alcohol at night that can really dry out as well on top of using PG.
Also, quitting the analogs puts the body through changes. She could be bringing up old tar and such from the lungs and might be coughing more from that. The body has to expel a lot of toxins and it takes a while.
She also could be getting a cold.
There's so many factors. If it's persistent and doesn't go away with another type of liquid like VG she might want to see a doc to see if she has a virus or something.
I wish her luck!