It has been 2 years since last time I crossed there but conditions should be much the same.
The crossing into Tecun Uman GT is from Ciudad Hidalgo,Chiapas,Mexico.
For tourists the crossing is the old bridge downtown , the newer bridge at the north of town is /was reserved for commercial trucks only.
Cd Hidalgo is southeast of Tapachula. There is another border crossing northeast of Tapachula ,at Cacahoatan which is more hectic in the bottom of a valley.At either crossing you will probably be chased by youths,on foot ,on horseback, on scooter, offering to be your guide through the border tramites- you do not need their help
For either crossing you should get your Mexican exit paperwork done at the customs terminal on Mex 200 about 8km west of Tapachula at the village of Colonia Viva Mexico. This is at the road junction for the bypass south to Cd.Hidalgo.Do it first !
After cancelling your Mexican TVIP and tourist card see how much daylight you have left.You are quite welcome to spend the next day in the border zone . There are hotels in Cd Hidalgo and Tecun Uman but Tapachula is the more interesting place to spend a night.Tapachula has a nice city center and plaza, lots of hotels , good restaurants and maybe free evening marimba concert on the bandstand in the plaza.
But you as a tourguide probably already knew all this.