I'm thinking of getting my girlfriend's friend a nintendo switch for her birthday
That's a lot of money. What is making you give her such a pricey gift?
Her friend had given me a blowjob
Does your girlfriend know about this? It is considered by most to be very wrong to trade sexual favors with someone who's not your girlfriend while you have one.
She doesn't need to know.
Your girlfriend will ask why you've bought her friend something so nice, and then your guilt may get the better of you. You should not buy someone a gift just because they did something for you, anyway. Gifts are meant to be given because you want to give it.