If the 4Runner came with a factory hitch receiver, the max hitch weight might be stamped on it. Otherwise check the owner's manual.
My '08 Lexus GX470 has a 6500 lb tow capacity, and 900-some lbs max tongue weight if a WD hitch is used. So even though max tongue weight is usually 10% of the tow rating on most vehicles, it isn't always so.
I'd suggest she look for a trailer under 24' long and under 3500 lbs dry weight. For some really light, easy to tow trailers with less frontal area, look at Scamp, Casita, and similar.