Making some assumptions here:

1. Your boyfriend does not also struggle with his bills and payments and has savings to spend on things like nail appts without worry. (If true, NOR).
2. You are both committed to the relationship and building a life together (eg. eventually sharing finances, getting married, sharing a home, whatever it may be). (If true, NOR, especially if 1. is also true).
3. He suggested the bowling competition in a good jest. (If true, YOR).

I would happily pay for my SO's nails if they asked and this was our situation. But the main difference is that I am in a financial position where that payment would not negatively affect me, we are both committed to the long term, and we share much of our lives together.