Um, probably the right play here would have been (1) wait until 24 hrs before FRA-IAD; (2) select seats online; (3) check in with TK for first segment.

The "24 hrs in advance for E+" clock applies to each segment, doesn't it?

Someone at the airport can do this. Your folks might be able to ask UA's Twitter team to assign E+ seats for their FRA-IAD segment; that team has some similar departure-management style abilities to an airport agent.

The path of least resistance here is to wait until the TK flight has departed, then get UA to assign a seat (phone or online check in).