The monster wouldn't attack the party
The charmed condition only protects the charmer, as it states:
- A charmed creature can't attack the charmer or target the charmer with harmful abilities or magical effects.
- The charmer has advantage on any ability check to interact socially with the creature.
However, charm monster doesn't just charm its target, it makes it friendly too:
The charmed creature is friendly to you.
This adds a social dimension to the monster's perspective, which means the monster must reassess the situation. The Roc is no longer dealing with pack of prey, it's now dealing with a pack lead by a friendly, and its behavior should change accordingly.
But lets really dig into what friendly means, as described in Social Interaction (PHB p185):
In general terms, an NPC's attitude toward you is described as friendly, indifferent, or hostile. Friendly NPCs are predisposed to help you, and hostile ones are inclined to get in your way. It's easier to get what you want from a friendly NPC, of course.
Harming the caster's party is not helpful to the caster, in fact it's actively getting in the way and contrary to what the now-friendly monster is predisposed to do.
Resolving Interactions (DMG p244) states much of the same, but with extra detail:
A friendly creature wants to help the adventurers and wishes for them to succeed. For tasks or actions that require no particular risk, effort, or cost, friendly creatures usually help without question. If an element of personal risk is involved, a successful Charisma check might be required to convince a friendly creature to take that risk.
The only adventurer the monster wants to help is the caster, but nonetheless attacking the party is the opposite of helpful for the caster. Leaving the party alone is a task that requires no effort, no risk, and no cost, it's basically the least the monster can do for the caster and so it should typically do so without question.
This is the baseline behavior for the monster once charm monster takes effect. The caster doesn't need to ask the monster to become friendly, it just is friendly and the two don't need to share a language for this to be true.
Still, the monster will continue to be indifferent/hostile towards the rest of the party. The caster is an excellent position to direct the situation towards a favorable outcome, especially given the advantage to ability checks granted by the charmed condition, but if the two sides have some unresolvable conflict then the situation could progress into a combat encounter anyway.
For example, if that encounter with the Roc happened in its nest, then ignoring the Roc's warnings to approach its chicks/eggs could easily spur the Roc's aggression and retaliation. Or alternatively, if the Roc were literally starving, then not eating the party is a risk for the Roc, and a successful skill check might be necessary to keep the Roc at bay.