I'm not surprised you feel drained 
You aren't being a coward at all, you are being a lovely friend, but please don't give more of yourself than you can spare, if that makes sense. You can't emotionally support her to the detriment of yourself.
You can't find the right words, because, sadly, there are no right words. I only wish there was, but just be honest and say that.
Has she been in contact with SANDS? There are some truly lovely people there, most of who are bereaved parents themselves and it may be better for her to speak to someone who isn't emotionally involved.
I know she is pressuring you to give her a time when this will all feel better for her, but to be honest there is no time limit. There will never be a time where she will be ok with losing her son, but there will come a time where she will learn to cope with it.
I would tell her that, right now, she need to concentrate on getting through the next hour, that is all she needs to do, slowly, very slowly she will notice that she can look to tomorrow, then next week and eventually she will be able to see a future again, tell her that you will be there to support her as much as you can but this path we are on lasts a lifetime so just to work through her feelings as they come up and talk lots.