She was correct, you should not share the flue, unless you want to destroy it and have possible collapse of the interior liner, then Carbon monoxide and smoke.
Now the better news, you can hire a chimney sweep to do the job right, clean the exisying flue you have anyway, and find out if the flue is even the correct size for the woodstove.
Then you may have to line the chimney with pipe, $1500, apx or more depending on lenght, and install a direct vent on the boiler to exit the wall nearby.Another $1500 or more depending on size, ect.
Woodstove installation also needs to be covered by your insurace Co, or they may not cover you if you have a problem.
Last, fire marshall or building inspector needs to pass it.