For context, it may help to review the stanza immediately preceding the one quoted in the posted question:
She now determined that a virtuous woman
Should rather face and overcome temptation,
That flight was base and dastardly, and no man
Should ever give her heart the least sensation;
That is to say, a thought beyond the common
Preference, that we must feel upon occasion
For people who are pleasanter than others,
But then they only seem so many brothers.
From this earlier excerpt, we see that Byron (as he has been doing for a number of stanzas at this point) is tracing the thoughts of Julia, a young woman who is trying to convince herself that it is the part of virtue not to run from temptation but rather to confront and master it.
In the stanza cited in the posted question, the pronoun she seems to refer to the hypothetical "virtuous woman" mentioned in the first line of the previous stanza—the one who is not afraid of temptation but meets it head-on.
In continuing to muse about the behavior of this imagined paragon of feminine rectitude, Julia poses another challenge: Suppose that the virtuous woman admitted to herself the possibility that if she were still free to choose—that is if she were not already married or promised to another—she might enjoy the attentions of such and such a lover (namely, Don Juan). What then would she think and do? Julia's answer is that she would quell those thoughts and be the stronger for having entertained and resisted them.
If this reading of the structure, plain meaning, and pronoun referents of the poetic lines in question is correct, the words "if still free" express in telescoped form the longer idea "if [the virtuous but now spoken-for woman were] still free [to choose to receive the attentions of an ardent lover]."