That will be bad idea, you should accept multiple altcoins because monero community is not quite strong right now so you may not get enough players. But along with bitcoin you can accept multiple other alts with inbuilt exchange features.
He probably only wants to offer Monero because of the anonymity aspect perhaps? OP, I think it will definitely be difficult to attract enough players with Monero alone, I would at least add some other altcoins to the website.
Dash perhaps?
What if he attaches a mini exchange to the gambling website where whatever Altcoin or Bitcoin you have in your possession could be exchanged for monero before you place your bet ? I think that will work because unlike ShapeShift you wouldn't be charged for the conversion but the amount will be in monero equivalent as it was in your other Altcoin.
Idea is good but other coins conversion to monero and play is very big implement it will be as for that the store should be ready to use it and the conversion price should be ready to change according to the exchange price. But if you can install this service then you gambling site will become more famouse
It's best if the coin moved to monero conversion with a very big play can be implemented so that the shop should be able to menggunakanya and change it with the foreign price so you can install this service to the site more convenient and reliable gambling while playing the gambling.
There's a list of gambling sites accepting Monero.