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I would check your outside vent cap to make sure that it is loose
enough to open when there is air pressure. Also get rid of the
flexible duct and use rigid for the bulk of the run. Home Depot
sells small rolls of duct wrap insulation and metal foil tape.

John Grabowski
http://www.MrElectrician.TV

Maybe the tenant needs to let the dryer run longer at the end of the
cycle when the clothes are almost completely dry so any water that
collects in the duct can be evaporated and expelled.

That point makes me think that the problem is not condensation allegedly due
to the dryer duct being along an outside wall (unknown if on the INSIDE of
the outside wall or on the OUTSIDE of the outside wall). I just think that
water vapor condensing on the inside of the duct is unlikely to happen under
these circumstances, especially since the at the end of every drying cycle
it is virtually dry (not moist) air that is being pumped through the duct.
And, as you mentioned, that would dry out any alleged condensation inside
the pipe. Another test would be to simple run the dryer for about 5 minutes
on high heat with no clothes in the dryer and see what happens.

And route the duct not so close to the cold outside wall?

It doesn't matter, in my opinion. I doubt that is the problem.

Does the problem occur in summer as well?

Good question. I would bet that it does. And, I would be that the
underlying problem is an air flow problem, maybe due to a defective blower
fan in the dryer, or a blocked or stuck-closed vent cap.

In my case, the problem existed during the summer -- although the occupant
never told me that the problem was going on for at least several months.
And, during the summer when I checked, the duct was full of soaking wet
lint -- all due to no air flow, due to the blower fan not working.