Assuming her end point in NYC is Midtown-ish and and she is close to the DC train would just take the Acela before I'd take the shuttle every day. On the best day for LGA/DCA, the Acela is an hour slower, but the whole thing is so uch more reliable and pleasant plus less "disruption" time (getting on the plane, laptops off because of takeoff, etc).

If she does go down this route, she needs to make sure she has a sub-2 pound laptop she can keep out on the long apron delays she'll experience on both ends.

Agree with the general comment of just grab a hotel. The shuttle can be significantly cheaper if you play booking games (nested round trips can be had for <$200) and Delta rarely goes after that, but if you do it every day for a year they may decide to smack you down - and same day turns are more like $500 than $150 - and at that point even the $400/night hotel starts to look pretty equivalent.