Couple of things I thought of. One, she's 40+ years older than MC in the current timeline, which doesn't bode well for them having any kind of relationship that they could have had if both were young. Two, she's the grandmother of a girl she knows is MC's best friend and recognizes Darci's underlying attraction/love for MC and doesn't want to screw that up by him finding out that she's the girl he started liking in 1999. Third, she knows all the ins and outs of time travel and how revealing actions in the past the MC has to do might screw up the timeline more. There are other things, like how her sister would react knowing that she has purposefully stayed out of her life if MC lets that slip, there are consequences in the present if past information gets leaked that should best be left unknown regarding time travel, etc.