Well as long as thier record is good, they can apply again of course, but as far as getting their origianal DOH seniority back thats not how it works. Perhaps if the resignation hasnt been processed yet, they have hope....but generally speaking theres not a "Grace period" so to speak to change your mind.
A better question might be now to start contemplating is: Would the individual now be better off re applying at United and starting at the bottom or sticking it out at the new carrier. It would seem the individual appears to think UAL is prefferable one of the two.

I am curious as to why just days later they think they made the wrong call......and what would say months later they wouldn't change thier mind again? Its a very dynamic industry, fleet and growth plans can change, domeciles open and close all in a New York Minute.
Scenarios like this (while confusing to 50 and 60 year olds) are likely going to increase in volume in the coming years as a matter of generational habit/preference/ and general indecisiveness.