LST is something you do during veg, technically you CAN do it during flowering, but i wouldn't call that LST...nah, I'd call that HST....High Stress. It could turn herm, airy buds, lower potency, dropping leaves early...hard to say what could happen.
Now for a PC grow, you should be growing mature clones, not from seed, or if from seed, an autoflower like lowryder. For example, I'm growing White Widow and Northern Lights...when the WW flowers, whatever height it is, pretty much stays that height through budding...the NL on the otherhand, doubles in height within about 10 days into flowering, even the clones. It's not a big deal for me, but I have a nice 6' high cabinet. You didn't mention if it's from seed or not, assuming it is..so I would suggest 'your friend' be ready to either chop it down or move it out of the PC case, because it IS going to outgrow that in a few more days, two weeks tops.