Welcome to the forum, sorry to hear your guinea is so unwell.
How much does she weigh? Whereabouts are you? Has the vet diagnosed a dental problem, if so what is the specific problem? Is she fully recovered from her pneumonia?
As has been said earlier, she needs plenty of food frequently - every 4 hours ideally, absolute minimum of 10ml but ideally 20ml per feed. No longer than 6 hours between feeds if she is totally reliant on them and is underweight.
I add bran to every syringe-feed to help the pigs maintain their weight but if there is a chance another illness is causing the weight loss, you won't make any progress with her, but it won't hurt to try something like bran, oats, corn etc. to help stabilise and regain weight.
Water has to be syringed carefully, aim for 20-40ml per day minimum, I find it works best to give a few ml of water in the middle of and at the end of a feed. However this has to be done very slowly and shouldn't be forced - if she really won't accept water by syringe and there are signs of dehydration then she may require subcutaneous fluids at some point.
Best of luck with her, please do keep us updated.