So while there are by the book rulings to this, as stated in diplomacy rules, there is also the leadership feat, which has additional rules for taking companions. These companions differ as they are far more loyal and the PC generally gets more control over them.
So what we can get from this, is they have a hostile creature that they are trying to make friendly. However, they also destroyed this creatures life (by the sounds of it) and the creature may have good reason to never like the party. This could be portrayed in ways similar to the bluff modifiers, where the more unlikely a lie is the harder it is to succeed, except in this case, its a matter of how much they destroyed this creatures life.
This could easily stack some large penalties, and may not be the fairest choice if you want them to actually have a shot at this.
I would probably make the rolls semi-scalar, having different results produce less and less desirable outcomes. Perhaps a 30 (and a well enough phrased argument) could make the creature a little less aggressive. A 15 may only convince it that it needs to do this to live. Then a 5 may only enrage it. Making no progress at getting through to it.
So we have a creature that hates someone, but has some intelligence, and probably wants to live and or get revenge. Now, you could have them roll and make the creature seem like its going along with them. Slowly gaining their trust. You could even let the creature soften up towards them if they consistently roll well, but its up to you to decide how long this takes.
If you take that route, you could have the creature pretend to take their side, while trying to sabotage them where it thinks it could get away with it.
Continued persistence despite its efforts, and or well phrased arguments or genuine remorse over their actions could help convince this creature they arnt as bad as it originally thought.
This creature in specific, may never warm up to a gnome or dwarf, as its hatred for them may be deeply ingrained in its core from social conditioning.