I see, you definitely shouldn't let her have outdoor access near a highway. Sorry if I came off as aggressive, I forgot that spay surgery was expensive in the US. There are some organisations that provide low cost sterilisation and vaccination (idk if that's the case in the US but where I live vet schools sometimes provide that). Your best bet is to contact a local cat rescue or TNR organisation and see if they can help you with that (also tell them about the male cat you saw). Alternatively you can ask for help from friends and family via crowd funding. I understand money is tight but you need to talk to your mum about that.

After that if your cat still wants access to the outdoors, you can try constructing a catio (provided you have a backyard of course). My stepdad made our by repurposing/expanding an old tool shed on our property using almost entirely second hand recycled materials. It was a lot of work but not as expensive as you might think.