Any hive swarms when it gets too crowded. I think there are two reasons that people may have more trouble with horizontal hives swarming:
1) The hive box is just too small. Think about the volume and number of combs you might find in a Lang at peak strength. If your TBH can't hold that much, then they are going to get crowded. Sometimes folks neglect to consider this, or forget about the volume that is lost in a KTBH compared to a TTBH. If your sides angle in, you'll need to make it longer!
2) Failure to open brood nest in spring. Lots of folks have trouble with swarming Langs after they plop a super of empty foundation on top of a box bursting with bees, assuming the bees will move into the space. Sometimes they do, but often they ignore it and swarm. Putting a bunch of empty bars behind last year's drawn combs is the same problem. The first flush of honey will go into drawn combs closer to the entrance, before the bees have drawn out lots of new combs. As the broodnest expands rapidly, it runs up against the honey area and the bees perceive it as congested. Instead, feed those empty bars in among the honey and brood bars (try every third bar in the brood section to avoid chillbrood until it's warm at night). And later, if you are running out of empty bars, harvest some of the honey so you can put the bars back in empty.
I do think that TBHs are more likely to swarm than Langs, but only if left alone. Remember that the bees WANT to swarm, and that "hands off" beekeeping and preventing swarms are contradictory practices. TBHs may need more frequent attention to be sure there is enough space. But you make up some of that effort in less heavy lifting!
The best thing would be for your friend to have an extra TBH too, just in case. Is there a particular reason that swarming is a much bigger deal for her?