In a recent session, we fought against a Lamia with charm person at will.
This is how we run things:
"You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn't ordinarily do.
An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing. Any act by you or your apparent allies that threatens the charmed person breaks the spell."
This, to our understanding, meant that the charisma check allowed for one order that could be very dangerous AND against the character's will.
The Lamia had we fight each other, because that's apparently in the scope of the opposed charisma check possibilities.
In addition to this, we also ruled that
" If the creature is currently being threatened or attacked by you or your allies, however, it receives a +5 bonus on its saving throw."
Was not referring to the "threatened" state as defined by the threatened squares rules, (i.e. you're in a square I can attack, thus threatened, thus you recieve a +5 bonus) but really was to be intended as verbally or factually threatened (i.e. "I'm going to kill you" or getting close with a raised knife)
So we didn't get the +5
We're not sure if we did everything right. One player was very unhappy with how things went and said that we should at least have got the +5 because we were technically threatened by the Lamia (in the sense that we were in her melee reach)
The Lamia however did never attack charmed creatures, nor her allies did
Question: Did we make mistakes?
I think you did everything spot on.
The notion of the +5 is to make it difficult to land a charm if the caster or what the target can perceive as the caster's group are acting in a hostile manner towards it. This could be violence, or just a heated argument. Either way.
It should be noted that Charm Person isn't mind control. It makes them see the caster as one of their best friends, but it does not preclude the notion that the party may contain other people just as important to the target. A GM would be every bit in their right to impose stiff penalties if trying to convince the target to go against their closest friends, but if they have no meaningful connection to the party they were traveling with, such as if they all meet at the Pathfinder Lodge, then it's nothing more than the Charisma check (and, depending on how traitorous the target, perhaps not even then).