It does not sound illegal to me (but I’m no lawyer). It appears the “hair shop” is passing on the cost of the transaction plus the cost of the machine plus a profit for themselves for providing the machine and service. If she truly only “rents” the space she should be able to obtain the funds from the customer however she likes. So perhaps she should look into getting her own credit card service or just refuse credit cards.

Usually things like this can be resolved by talking to the owner and laying it on the line.