Am I jumping to conclusions?

So my girlfriend is a junior and I’m a sophomore. She’s been excited to go to prom. Her friend just bought her ticket and they are supposedly going together. She wants my girlfriend to wear a tux and a vest to match the color of her dress. I can’t go because my girlfriend is broke so I have to just sit back and watch. I don’t want to jump to conclusions, but this doesn’t seem right to me. Her friend is bisexual and they work together. I trust my girlfriend and love her so much, and don’t want her to be held back by me. Any advice that anyone could give me? It would be very appreciated.