I do Eagle -> mainline connections at ORD with some degree of regularity. I can't remember ever missing a connection due to a delayed Eagle flight.
In fact, I fly Eagle fairly regularly, and I'd say that my on-time percentage on Eagle flights has been the same or perhaps slightly better than mainline. Eagle can tolerate a little more lateness on either end than a mainline flight, because there are far fewer passengers on board, so boarding/deplaning takes much less time.
(One percentage that has been higher for me on Eagle is the percentage of flights needing bump volunteers -- winter weather can wreak havoc with weight restrictions on smaller planes. If your friend wants to avoid this, be sure that she checks in online or shows up to MSP with plenty of time before her flight.)
Personally, I wouldn't be concerned about a 54-min connection at ORD at all. I've made it from the end of G (Eagle terminal) to the end of L (furthest terminal away from G) in 25 mins without even hurrying, so I'd expect that a 54-min connection would get me to my mainline flight in time for priority boarding
I wouldn't even be concerned about the delayed flight that arrived at 8:38; I'd have to hustle a bit, but I'd expect to make a mainline flight at 9:02 (especially because they would likely hold the 9:02 flight for a few mins for connecting passengers, considering it's the last flight of the day).
I would recommend that your friend bring only carry-on luggage that will fit
on Eagle. If she needs to check a big bag, that's fine (sounds like the worst that will happen is that she might have to do without luggage for a day or so, but at least she'll be in Hawaii...), but she should try to avoid Eagle's gate-check. Not that there's anything wrong with Eagle's gate check, but avoiding it will get her going about 5-10 mins faster at ORD.
And yes, if she's really concerned, she should just stand by for an earlier flight out of MSP.