He may not say she is headstrong but while he does try to make soem excuses for her behaviour, it is clearly wrong and he doesn't avoid saying that. IIRC she really wanted to go to Bath and while he was a little uneasy, he allowed her to go. And there's no evidence that it was her friend who persuaded her to go "ranging about", presumaby without a maid or chaperone, it might just as easily have been Eliza who said "lets have some fun, go out on our own without the maid, your father wont know..." and they both took to going out, mixing too freely with people without soemoene to look after them.
And it was Eliza who fell from virtue, not her friend, however wrong minded the friend was...
So I dont see that it was necessarily "peer pressure" that led to Eliza's seduction. Even if she were pressured, she was quite simply wrong to let herself be seduced and Brandon admits that, saying that her situation is different to Marianne's. Mar has done nothing wrong and need not reproach herself, whereas Eliza IS to blame to an extent, for her seduction which will problaby ruin her life.. and damage that of her child.