In Ch. 3, the silent knight knelt to pray at the casque (helmet).

Frederic reveals to Hippolita lines on the blade of the gigantic sabre;

'Where'er a casque that suits this sword is found,
With perils is thy daughter compass'd round :
Alfonso's blood alone can save the maid,
And quiet a long-restless prince's shade' . (ch. 4)

A modern translation from Spark Notes dost goeth;

Wherever the helmet that goes with this sword is,
There will be your daughter who is in danger,
Only a man from Alfonso the Good's line can help her,
And destroy the Prince. (Manfred)

I hope it's only Frederic's physically weakened state that makes him easily manipulated by Manfred and he'll recall his mission !
Oh, Isabella to be sacrificed to Manfred afterall ? !