Need to consider the age they're considering and their budget. Some manufacturers of HTTs are no longer in business or have been merged with another company who does not make HTTs. Suggest they look at ones who have a longer track record, still making HTTs and have better reputations for support. Rockwood ROO/Flagstaff Shamrock - same trailer, different name - come to mind because we had a ROO and no issues. Ours was a 2007. Jayco has a good reputation and build a quality product as well. There are others, but these come to mind first.
The older the HTT, the more chance of leakage and water damage, so they need to check this very, very carefully. Our '07 had no water problems. HTT camping is a great start. Hope they can find the perfect one. A 19 is a nice starting point as well. Good luck.