Here's an update on this situation. After applying Chaindrite a few times, and thoroughly cleaning with a vacuum cleaner and washing clothes in batches my friend is still plagued by insects, although their numbers have declined.
She contacted Rentokil, who came to inspect and took away samples of the insects. They have said they cannot identify the insect, which seems rather strange. However, they have offered her a 10,000-baht package which I believe involves three or four spray treatments, of which at least the first would necessitate vacating the room overnight. Is it worth going ahead with such a treatment if they can't identify the insects?
She also contacted CNX Pest Control, but they said they couldn't help and told her to ask a cleaning company. Maybe they only deal with people who live in houses and not those who live in rooms in a condo.
Regarding Dellboy218's suggestion to use diatomaceous earth, does anyone know where this product can found in Chiang Mai?
Thanks once again to everyone who has made suggestions on this thread.