I like to be more precise in discussing these kind of scenarios...generally the rules are written using the term "Leewward boat" to indicate the boat that is "downwind" from the other "Windward boat". Saying "downwind" boat can be misleading since a boat can be sailing to windward and still be downwind of another boat. And a boat sailing downwind can be upwind of another.

In the described scenario...the windward boat is sailing downwind (or off-wind) on port. The leeward boat is sailing to weather (or upwind or close hauled). Windward must keep clear. But, both boats have the obligation to avoid collisions. If windward fails to take action to prevent leeward from having to avoid her, then windward has fouled leeward and should take a penalty or be protested. It is a good idea (though not required by the rules in this scenario) to hail the give way boat well beforehand when she can still take avoiding action. Leeward will need to avoid the collision...with her recourse to protest...which will hopefully convince windward to make their penalty turn(s) according to the sailing instructions. Otherwise, filing the protest with the R/C will be required. In a protest hearing, leeward will need to show she waited until a point where there was no safe choice other than to avoid windward to prevent the collision. This can be difficult in some top fleets to prove...so knowing the wind speed, boat speeds, at what point(s) you hailed, closure rates, distance (in boat lengths) when evasive action was taken will all bolster your case. The onus should be on the giveway boat to prove she did not foul you.