Parging foundation in Rain
There are no issues if you are using mortar.  In fact medium temps and a little dampness is no big deal and will allow the mortar to cure slower resulting in a stronger bond.  I am not saying that you do this in a heavy rain but no need for tents unless it is monsoon season there.

However if you are using some sort of epoxy the walls need to be very dry and again tents won't help because you would have to keep ground water out too.  Then you would need to do this after 4-5 dry days and sometimes even more - all depends on ground water drainage around house.