This may not be the problem, but... I purchased pens about 7 years ago, bulk order of 50 pens with up to three lines of text, blah, blah, blah...
Upon ripping one apart several weeks ago, I found the same innards as a slimline pen. Same size parts all the way through. But like your friend, after about a week of writing with a new one (right out of the pack), it starts to retract during usage.
It was the transmission.
When new it sort of locks into place the same as any slimline you are used to using. But after a bit of writing with it, the transmission doesn't hold anymore. It slowly turns as you are writing and allows the refill to retract into the pen.
I've since taken the ink out and discarded all the pens. Cheap innards was the cause.