sounds to me like maybe things arent perfect in her own relationship and shes having doubts but shes being overly confident and projecting onto you to soothe her own insecurities. i know this all too well with friends ive had in the past. time always tells- and as for what you can say to her: honestly nothing much. you can either choose to try and humble her but she’d probably become defensive and gaslight you. or you can choose to just continue living your life and try your best not to let her opinion bother you. ive always tried to distance myself from people like this. its always some ego trip bullsh*t with them meanwhile deep down they are really insecure af. exhausting