OP - Nobody here can help you and nobody at LH will even talk to you about the issue without having the passenger who had boarding denied on the phone. Dealing by Twitter, email or other indirect method is a terrible approach when you need interactive communications. That is accomplished by telephone or in person. Nothing else.

Why exactly was the one in law denied boarding? Presumably there is a perceived document problem based on LH's interpretation of TIMATIC. So, focus in on the difference in the situation of the two.

A link to TIMATIC's data entry page is cut & pasted below. Fill that in both for the passenger who was denied boarding and for the passenger who was permitted to board and then figure out what went wrong.

If LH has simply misread TIMATIC, you may be able to help to walk through explaining the issue. But, nobody can say that LH has made an error until you explain here what the issue is.

If the passenger was wrongfully denied boarding, EC 261/2004 compensation will likely be due, but my suggestion is not to focus on that at the moment.

Link to TIMATIC:
https://www.united.com/web/en-US/app...aspx?i=TIMATIC