The reason I think this is not possible is because;
1. One can only know of a transaction with the least fee rate after the block has been confirmed. No one will be able to guess the exact range of Bitcoin fee rates for the transactions to be confirmed in the next block especially for the low rates.
2. The randomness of interval between Bitcoin blocks. The longer the next block takes to be confirmed, the higher the fees increase, for example
I think it is possible to make what he asked.
I have a similar website which shows mempool data, and I liked the idea. I will think in a way to implement it.
As I have mempool data, I know what is the mempool size. The mempool size now is about 67mb, which are about 60 blocks.
I can send him an email when the mempool is about 10mb, which is about 10 blocks. Or 1 mb, what he asked (nearly empty). When the mempool is similar to the blocksize you can get confirmations with 1 sat/byte.
It not 100%, as you pointed out. Nobody know if a 1 sat/byte will get confirmed in the next block (because it may take longer than 10minutes). But it is very likely to get a confirmation with 1, or 10 sat/byte if you want to be conservative. It is an amazing estimation, and if he just wants a warning when fees are low, I can make that.
I just want to think what is the best way to alert the user: email, desktop notification...
Once it is done I will share here