Preventing bullying!

Hi, when we got our second dog, myrtle, Dan really bullied her. She is almost a year old now and the bullying had stopped and Myrtle had begun to assert herself a bit more. However, yesterday she was spayed and Dan is really bullying her again, taking advantage of her being somewhat more vulnerable and as she feels vulnerable, she is responding by cowering and running away from him. What I'd like to know is a) am I reading this right? Is it bullying? and b) what is the best way to deal with this?
Prior to the last month or so I had believed that Dan was a dominant dog, but since reading some of the posts on here as to how a dominant dog will behave, I think it is more likely due to his insecurities; he is very insecure in general! What I'd like to achieve is showing him that bullying is not acceptable, but without making him more insecure which I think "telling him off" would cause. Does that make sense?
All suggestions/opinions welcomed!
Jo

If your bitch was spayed yesterday she needs somewhere quiet to recuperate, and also to be protected from the other dog who may open her wound. I would separate them for now, and give the bitch a safe place to relax. I`d say also be careful when she gets her strength back as she will be subject to hormone changes as the body adjusts. Forget about who`s the `top dog`. It`s your house and you`re the leader and you set the rules.
ETA I wouldn`t tell the dog off - I`d substitute a desirable behaviour - if he`s bullying, ask him to do something else then reward it. Dog feels good, behaviour has been interrupted. Any attention given to the bullying behaviour will perpetuate it.

Thanks for responding, he only growls quietly at her, he doesn't pin her down or anything like that. It's too fast to distract him, one quick growl and it's over. She's spending most of her time asleep at the moment, upstairs on our bed! Right now she's on the sofa, watching him and he's watching us!
Jo