Just in case this haven't been able to find those rare drops in the month~ since you have posted this, I will offer some tips that might help you and others figure out where those 13★s are.
First off, as @GM-Deynger said, find out if they have the notification settings set to 10★ or above or 13★ and above. Those settings are here. Also I recommend the settings I have here.
If it is on 10★, they may have been getting it mixed up. To be clear it is the rainbow text AND the rainbow box spinning on their screen right? If it is only the "Rare Drop" text that is rainbow, then that means she received a 10★ weapon.
Here's a photo comparison:
Make sure they have checked their storage as well! 13★ will go there if their inventory is full at the time of the 13★ dropping (assuming they had the Immediate Acquisition setting on).
Also note that those items can drop untekked. And you will not be able to tell which of those are 13★ until you reach the tekker to appraise them. They will show up rainbow when you go to select them as seen here.
If you can't find them after that, then perhaps there is actually an bug causing it to show up.