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First, when you say "edit", is he trying to insert someone in that lineup spot? If so, it sounds like an IR issue (meaning you shouldn't try to fix it for him). Who does he have in the IR slot(s)? If an ineligible player is in one of those spots, he probably has to fix that by moving the player to the bench.
If this isn't it, tell him to go to his team page (desktop version) and switch from swap mode to classic. This can identify or even fix the problem.