Yes I completely agree with you in this case. However in this case the sorcerer just comes back again and again to try assassinate the PC's. She does nothing else or extra.
So I have two choices:
1. Players get XP for her just because they survived. They can get the full XP for her several times as long as they keep on being ambushed.
2. When she is finally killed and no longer tries to assassinate them, full XP.
In your example the players overcame the mission which warrants XP. But if the mission is a failure, no XP. So the question here is the mission to survive the ambush OR prevent BBEG from ambushing again. The question is also what kind of behaviour do I reward. I want to reward the players when they slay their opponent or stop her from doing bad things. Just going around after her and not stopping what she is doing is something I don't want to become as a XP-factory.
Per RAW each encounter is "different" so full xp if you defeat the opponent or overcome the obstacle each time. Your decision as DM as whether this was done or not. Before each encounter you need to decide what the "goal" is that the PCs are trying to accomplish - somtimes it is merely killing (or otherwise defeating the opponents - capturing them alive should always count as defeating the opponent).
As far as getting full xp the next time they face the sorcerer - has her CR gone down yet (hence despite granting full xp for the next encounter her xp total is less)? Will they have advanced levels by the next time they face her (potentially less xp gained). Per RAW she could have lost a level due to the death of her familiar and in addition to that, she won't have a familiar the next time they face her (unless it is a year and a day later).
As far as the familiar alone goes - remember that upon the death of its master the familiar loses its "magical beast" templete and all benefits of being a familiar and reverts back to a normal creature of its type (with normal creature's hit dice). Familiars never gain hit dice - their hit points change according to their master' but they themselves never gain xp.
from the SRD
"A familiar is a normal animal that gains new powers and becomes a magical beast
when summoned to service by a sorcerer or wizard. It retains the appearance, Hit Dice, base attack bonus, base save bonuses, skills, and feats of the normal animal it once was, but it is treated as a magical beast instead of an animal for the purpose of any effect that depends on its type. . . .
A familiar also grants special abilities to its master (a sorcerer or wizard), as given on the table below. These special abilities apply only when the master and familiar are within 1 mile of each other. . .
Hit Dice: For the purpose of effects related to number of Hit Dice, use the master’s character level or the familiar’s normal HD total, whichever is higher."
