Personally, I wouldn't take the passive-agressive approach with a woman like that.

I would tell her straight to her face her dog does not belong at a dog park if it's agressive to other dogs. I would also do my best to get contact info for her. If her dog was to hurt any other dog, she is liable and that should be made very clear to her. Thabeing said, I would never enter the park if it's just you, her and two dogs. ALWAYS make sure others are there to witness any acts of agression from her dog in case their are any injuries and someone has a phone to call police in case of injuries and confrontation.

Also, I would try to figure out if she has a pattern of when she goes to the park (eg: everyday at 5pm or saturday mornings at 11am) Also find out if the city has someone regulating the parks (eg: park rangers, police, etc...) and let them know you and others are having problems with this one dog.

I do have to say that I'm proud of that dobie owner. He's willing to work on the problem and is doing it appropriately! I hope he's also working with a trainer to assist with the aggression issues.

I do give my dogs time outs in the dog park. If they start playing a little too rough, I do take them out of the situation move them to an area of the park where there are no other dogs and have them focus on me with some basic sit, down, stay, come, lay down commands and favorite treats! After a few minutes of the time out I re-introduce them to play. I will leave the park if it becomes to rough and a couple timeouts haven't corrected the behavior.