Just going to "talk out loud" here for a minute for the purpose of problem solving...don't think I have a strong opinion as to what is going on yet.... Let me know what you think....

1. IF THE POP WAS IN THE HIP (i.e. the hip joint)
Could it be possible that the ITB "flicked" over the greater trochanter? Could the pain be related to irritation of the ITB / trochanteric bursa / muscular attachments in the area... Resultant muscle tightness could be due to compensatory factors?
I suppose you don't know what her running technique BEFORE the injury (unless you saw her before that race). If she was somewhat weak throughout her gluts / core and had trendelenburg type of gait while running it could be possible even if her ITB isn't "tight".
Questions: Where was the pop? Any tenderness over the greater trochanter? Any discomfort with testing ITB length (Ober's)?

2. IF THE POP WAS IN THE GLUTEAL AREA
Potentially could be a tear in a muscle however I guess I wouldn't suspect it unless she had a previous injury / was running with pain in the first place / was running with a healing injury.... (I say this because I would suspect a tear in any of the muscles in this area would take a large force (trauma) or a sudden eccentric force (jump / sprinting) rather than repetive motion at a slower pace (ie. marathon).....but again - this is just a suspicion...could be wrong)

3. IF THE POP WAS AT THE Wikipedia reference-linkSIJ
Again, my gut is saying this is unlikely but again...I'm not the one looking at her. Did you stress test the SIJ and the pubic symphysis?
Question: Does the SIJ belt help (ie. does it allow her to walk further; is her pain less...)? If it does, it doesn't necessarily mean there is instability and / or dysfunction at the SIJ...it could mean that it is just assisting by backing up some of the "core muscles".

Anyway....since it has been 5 months since the original injury maybe the initial injury doesn't matter as much at this point (unless of course there is some instability at the SIJ / pubic symphysis)

I think I would, similar to you, be dealing with the myofascial tightness to try to normalize the tension in the area. Along with this it would be important for the client to be doing stretching (QL, quads, gluts, hams, iliopsoas) and strengthening (gluts, overall core) daily to sort out any muscle imbalance. With her walking, ensure she is maintaining good technique and not fatiguing in to a trendelenburg gait (although watch her and see if her gait changes once her pain comes on).

What does she do for the rest of her day...i.e. at home / work? Is there anything else that could be playing in to this?

Interested in hearing your thoughts.....