Just to get it out of the way, I'll note that disadvantage on the save should not be possible in this situation since the PC already has advantage from the hostility, so disadvantage imposed would just cancel that out and the PC would roll normally. I get that your DM is already fiat-ing the spell's duration, but I don't know that this part was ignored intentionally.

So... when done by a DM, I'd compare this to a ghost's possession ability. Both cause an indefinite loss of control for the PC. Ghost is a CR4 monster, so it's within the realm of a tier 2 party to face. So, similar to the Ghost's possession, there needs to be other ways for the effect to end.

If the DM is using the spell as written (other than the duration), there are ways of ending it — even for a tier 2 party. There's the rather obvious "do damage to them so they repeat the save", there's Dispel Magic, and anything that will end the charm condition (e.g Calm Emotions will suppress the effect while active, or you could find/pay someone to cast Greater Restoration on them to end the effect entirely).

It is quite an adversarial thing to fiat an indefinite Dominate, but it's not mechanically outside a DM's toolkit or beyond a party's capability to deal with is how I would describe it.