I understand how you feel - sometimes, when I'm feeling 'down', it's like you know that you need a 'recharge' of your emotional batteries before you can handle the problems, or even the successes of friends. You just feel drained, and want to shut down.
On the other hand - you know that you NEED your friends - and that it takes BEING a friend, to have a friend be there when you need them.
When I run into that kind of situation, I know I need to call my friend -but I also know that the call will probably push me into overload - so I set up a 'backup' plan. I'll set a timer, ring the ringer on my cell phone, ring my own doorbell, or tell my son to come and get me - I'll create some reason to "interrupt" the call - so that I limit my time on the phone.
I've still acknowledged my friend's need for support, and I have protected myself from becoming even more "down". I WILL call her back for a long discussion at another time when I'm feeling better - but this way, nobody gets ignored or hurt.
You might try this with your friend - it will acknowledge her happiness, while limiting your exposure to it and might save your friendship.