Has he ever been around a puppy? Are you willing to separate the dogs and rotate which is out, if things go south?
A warning about dobes, because I have one – they are NOT gentle with littles. You’ll have to work very hard on teaching a dobe puppy to be gentle with smaller animals – our girl is nearly three and still occasionally stomps my cat. They’re also tough puppies and are quite literally a full time job until they’re around 2 years of age…I’ll happy talk more about the breed if you want, but I don’t want to flood this particular answer with my babble. 🙂
Fancy rats are prone to tumors, and I’d be willing to bet that’s what this is. Please get her to the vet TODAY about this.