Most modern phone/tablet chargers are switched mode power supplies and can accept a huge frequency range of input power. They do not have any problem at all with 400 Hz and work just fine. However, they are not officially certified for that use, nor are the power outlets.
It might be a customer mod, but our in seat power outlets for passengers have 110V with 60 Hz, same for the power outlets at each pilot seat, the maintenance outlet in the flight deck, same as the vacuum cleaner sockets and the medical outlet in the cabin have 400 Hz with 115 V.
We have to use the low frequency outlets for our iPad chargers, however the reason, according to our technical staff, is not the frequency but rather a grounding problem with the 400 Hz outlets that can lead to very high residual voltages and a high risk of electrical shock. For our former EFB we used the 400Hz socket, but needed specially protected power supplies to prevent that risk.