It sounds unusual for the mechanic to have not tightened it correctly but it is a simple fix. I also trust that it is just the arm fixing loose and not the actual spindle itself.
Remove the bonnet.
Turn the ignition on and activate the wiper, as it moves to the top of it's travel turn the ignition off, this will leave the wiper parked in the up position and expose the lower spindle cap fully.
Prise the black cap off (you will find a small recess for this) and tighten the nut underneath with the correct size spanner/socket, it should not need too much.
Test wiper operation.
Replace Caps and Bonnet.
Have a cup of Tea.
