Nothing. We could say that almost every other altcoin are fast and cheap, but it's mostly because almost no one uses them. Hence it wouldn't make sense adding a cryptocurrency payment option that wouldn't be used in the first place.
An option: though adoption is low, you could try adding lightning payments. There's a bigger chance of someone actually using it.
Or may be he can just use the stabelcoins for the payment? Considering their nature of stable and fast transaction speed one can rely on that. Later the coins can be swapped to the bitcoin or whatever favourite coins OP has.
Idk, if there is any system which converts the coins that are received on particular address directly to the bitcoin, if it exists then OP can option out for that. May be like swap portals? But they change the address for each transaction I guess(!)
Adoption needs to be done, way out needs to be found out for the better future of crypto's. Otherwise they will be just nuts lying on the floor.