You can add more payments as you wish, it will give the buyers or the shoppers more options for their payment. If your items' price are a bit high, then bitcoin payment method isn't a problem at all, even its volatility.
It could be a very good idea to add Bitcoin as a payment method. However, the guy should have compare all pros and cons of the decision. If it is a private store, and the fellow owns it, it will be much easier. If not, I would rather think about adding BTC — maybe, not.