C is only "forced" to pay a higher fee if they want to have their transaction confirmed fast, otherwise they can wait until it is confirmed. and in real world scenarios the trade between the two party pauses until the transaction is confirmed and the "sender" should pay appropriate fees otherwise the trade (or whatever the relationship between the two parties is) can be nullified.
to answer your question, @TryNinja already said what C could do. it is called CPFP which in other words means C pays a high transaction fee that not only pays a high fee for this 3rd tx but also covers the additional required fee by the previous 2 transactions. (txAtoB.Size + txBtoC.Size + TxCtoC.size * high_priority_fee_per_byte)