Cat was in heat now she's acting very lethargic and sick

Hello wondering if anyone can help me or point me in the right direction. Cat was in heat for the past few days now shes lethargic and kinda cold. She was vomiting earlier but she's starting to show signs of improvement. This isn't my animal it's my girlfriends cat and I'm she should take it to the vet but with it being the time of the month for all the bills it isn't an option. Any idea of what's the happening with the cat and maybe what I could do at home to help remedy the situation and make her more comfortable?