Welcome to CatForum and thank you
for helping to love and care for the forgotten ferals. I tame, socialize and foster feral, semi-feral and poorly-socialized kittens and cats for adoption. I find it to be very rewarding work.
Ideally, the female should have been spayed before she came in season. In fact, most TNR groups spay every female cat when trapped, regardless of pregnancy or not. The reason isn't a heartless one, it is simply one of expediency ... the only way to not add to the feral numbers is to prevent more births and many times the TNR groups are unable to re-trap a wary cat so it becomes a little bit of an opportunistic spay 'em while you got 'em attitude. You also need to be aware that unless you are able to isolate this female into a cage of some type, it is almost certain that you will *never* have an opportunity to spay her while she is not pregnant. Cats can come into season, breed and catch pregnant within days of birthing and her pregnancy will develop even as she is nursing her current litter of kittens, beginning a non-stop cycle of pregnancy, birth and kitten-rearing.
If you would like to try to help her with her litter, you could provide a nice nesting area for her on you property.
Here is a link to a topic about pregnant cats and birthing preparations.
Also, check out the stickies for "shelter ideas for feral cats", there are some great ideas.