It depends on what sanctions she has been playing at the 14u level and what sanction she is being invited to play at the 12u level. If she has been playing 14u ASA she isn't supposed to be eligible for a 12u ASA event. If the tournament in which she is going to "guest play" is sanctioned by another entity such as NSA or USSSA, to my knowledge there is no problem.
Even if it is a step down from one ASA age group to another, as long as the lower age group event is not a qualifier for ASA Easterns or USA/ASA Nationals, the issue is not likely to be raised by anyone because TDs in non-qualifier events rarely require rosters to be turned in. For ASA qualifiers for Easterns or USA/ASA, rosters are required and could be used to challenge eligibility if your DD ends up playing in the Easterns of USA/ASA.
Just be aware that if someone is really out to get your daughter and they have proof that she played in two age groups in ASA events, they could challenge her ability to play in future ASA events this season.
I am not familiar with the rules from other sanctions on this, but I seem to remember someone posting that for USSSA, which has players rostered on-line, the rule prohibits the player from being on rosters of two different age groups.
