There are two ways to unlock the option:
1. Have at least 70 Affection with her and talk to her about the Phoenix's status for the 5th or greater time. The version you get from this, she'll refuse.
2. Start her second quest, but after the ambush don't talk to her for the next 168 hours (seven days); this will cause you to 'fail' the quest, but Saendra will still be around afterwards and this time she'll accept your help.
These two are technically mutually exclusive, as her Affection is capped at 69 (i.e. point 1 can't happen) until you either successfully complete her second quest or give her help via point 2 above (which requires 'failing' the quest). It's possible to meet both conditions, but you'll have to do the second route first which blocks off "Offer Creds" for good.