As I read it, the FP player needs to either add 3 threats or select an opponent who is capable of controlling a site, and this opponent is then allowed to take control of a site. The FP player must choose an action that can be fully performed.
The question then is whether the action is fully performed when the Free Peoples player chooses an opponent, or only when the Free Peoples player chooses and opponent AND that opponent has had the opportunity to take control of a site.
Note that while it's tempting to say that the opponent the Free Peoples player chooses must be "an opponent who may take control of a site[,]" it is clear from the context that the phrase "who may take control of a site" describes what the chosen opponent may do rather than explicitly limiting the choosable opponents to those "who may take control of a site."
The text itself seems to imply that the Free Peoples player only has to choose an opponent, and his/her work is done. However, the general import of the card seems to require that the the FP player add threats or let an opponent control a site, and the "choose" language is in place solely to address the situation where there are multiple Shadow players, while the "may" language is in place in case the chosen opponent doesn't wish to take control of the site.
Thus, while a literal reading of the text seems to allow the FP player to choose an opponent even if that opponent is unable to control a site, this seems to be a technicality at best. A tournament director could rule either way, although I think the interpretation that the opponent must be able to take control a site is probably the better one.
Ultimately, in friendly games, just get this agreed upon ahead of time (I don't think it matters which way), and in tournaments, you might want to ask the tournament director if you're worried it might come up.