jaydee, welcome. thank you for being such a good friend. you are correct that losing the other dogs will be depressing. your friend is already there. there ARE ways you can help, and bless you for being willing to do so. please know that it isn't going to be easy, and she may turn her anger on you - because it's safe to - for a little while. be sure you don't take that personally, okay?
you gave a few clues to some serious, deep problems your friend hasn't resolved. as you said, you're availabe 24/7, yet she feels alone, and a dog is her best friend. this tells me that something awful happened a while back that she hasn't addressed yet. something that makes her mistrust, maybe avoid, people. she has trouble with relationships. whatever it is is going to take more than a listening friend with broad shoulders. the best thing you can do for your friend right now is get her to a professional counselor. she won't want to go. she'll think talking about it brings it all back, when in fact it's present every minute of every day. you may have to do an intervention to get her there. you may have to take responsibility for transporting her there the first few weeks, til she can do it for herself. the loss of her dog and her dad also must be addressed. grief is hard work. i suspect there have been losses in the past she hasn't processed. doesn't know how. the counselor will help.
with all that, being willing to listen, not be judgmental, and continue to love her - these are the gifts you bring.
please visit the bereavement page for articles on this very dilemma. there you will also find a way to contact me personally. i will walk this journey with you, and be YOUR back up as all this unfolds.
your friend is a very lucky lady to have you in her life.