Is it justified that I feel upset that my crush is hanging out with my friend?

So this guy told me he was interested in me and I was interested in him too. Then after we had sex he told me he wasn't ready for a relationship.

Due to the lockdown and haven't been seeing him for a couple months made my feelings for him fade. My friend knew how I felt towards him and our situation but I did think that she might be interested in him (she always try to talk to him and suddenly took interest in cooking because he is into cooking)

We had a small gathering last weekend and after I left apparently she stayed there talking to him til 3:30am.

Suddenly now I see a snapchat that they are cooking food together. I don't know how I should proceed about this. Technically they are both single and can do whatever they want but I feel betrayed

We are all in uni, he's 23, I'm 21 and she's 19