Does he have the title? I think that would show the year. I'd be careful buying a trailer without a title. Other members have had great difficulty registering trailers without titles.
Model 17572R should be a 17 ft "Traveler" model year 1972. Here is a link that shows the brochures for that year: https://hilotrailer.com/home/then#1970
Click on the year 1972 and you should see a page showing the one he has.
Hi-Lo 1707T - Tire Minder TPMS on Tow Vehicle and Trailer, 300W Solar Battery Charger, Equal-i-zer WDH, Progressive Dynamics Converter, Fan-Tastic Fan
2014 F150 Platinum 4x4 3.5L EcoBoost SCrew