Furthermore, if his family's living expenses depended on the sales of the tailor shop, I don't think his mother would accept that offer. There are many ways to earn bitcoin, and I also think OP should focus on ways like we are doing rather than giving advice to his mother.
Accepting that offer is not bad but it is how deep that tailor shop treasury is. Because it should be considered as an investment by using part of the tailor shop income to store it in bitcoin. I am sure not all customers of the tailor shop will use bitcoin for their payment.
OP must plan to not touch these bitcoin too often because each time, it will cost transaction fee so reducing times to move bitcoins is better.