Shopping trip with my gf's slutty friend

My gf has a birthday coming up, and one of her friends texted me asking to take her to the mall to help her pick out a gift, and obviously not to tell my gf so it's a surprise. So I took her to the mall, and it didn't take too long to find something. But she wanted to do her own shopping and I was taking her around for a bit. At one of the shops someone assumed I was her boyfriend and she loved it. She started playing along, calling me her bf, putting my arm around her shoulder, stuff like that.

She kept up the boyfriend thing for a while, until we passed by a lingerie shop and she wanted to go in. She was asking me what she'd look sexy in, and we eventually picked some red lingerie. Then she said I'd get to see her in it if I bought it for her, and started giving me puppy dog eyes. So I paid for it and we left to go to her place.

I waited on her bed while she got changed, and she came back to the bedroom in the lingerie. Gave me a little show, then started grinding on my lap a bit. She could feel I was hard and started rubbing it over my jeans, asked if she could "try" it. Wasn't sure what kind of trying she wanted but I told her she could, so she pulled it out and started sucking it. After a few minutes of that I fucked her good in her brand new lingerie.