They're very friendly with their family, and she'd enjoy the company of your dog. Bullmastiffs tend to not like strangers (both humans and animals), but nothing a good socialization won't fix, so there's nothing to worry about, go get a bullmastiff, you won't regret it, but note that they're REALLY LAZY, lol.
Any dog can have issues with their housemate so there is no guarantee. I would say typically they get along together nicely. Bullmastiffs many times had to work together which meant they needed to get along. They couldn't be fighting each other or bickering.
I've some experience with them, all mostly positive. They are great dogs although they can have some gas, but that isn't hard to put up with. They can also be a protective breed still having some of their instincts intact. With training and socialization they can be stable and friendly dogs, this doesn't mean someone can enter your home uninvited or when you are not home. They very well may do their job without an training specifically for guarding. You should always keep them in a secure place like inside and have a good fence.
The gas problem of the Bullmastiff ain't THAT bad from my experience, my boxer have much worse gas problems than the old Bullmastiff I used to have.
They are great dogs, and require very little grooming, but here are some things you should know about this breed before you buy:
1) They slobber, and I mean, THEY SLOBBER, as in you have to keep a rag lying around in each room to wipe their saliva.
2) Some gas, not as bad as Boxers though.
3) They snore, quite loudly, which could be a nightmare for you if you're a light sleeper.
4) Bullmastiffs would usually get up every 2 hours, take a walk around the house, and make sure everything is ok, if they find a toy however, it's party time at 2am!
5) They aren't Labradors or Border Collies, they aren't exactly the easiest dogs to train.
6) They can be quite dominant, only get one if you are capable of maintaining a pack leader status.
7) They are very low energy dogs, too much exercise can hurt them, if you want a more active dog, get a Dogue De Bordeaux.
8) They can be quite protective, and needs plenty of socialization.
9) This is quite obvious, but should be said for the record: They can be quite costy, they're massive dogs, thus requiring more food, and larger vaccination quantities, which translate to more money from your pocket.