I think it's lovely that you are trying to find the best way to support your friend.
My daughter was stillborn at full term 15 months ago. It does change you. It destroys your faith in your body, your confidence as a woman, and can leave you with a lingering sense of shame and guilt and failure. These are all utterly normal, almost textbook, responses to what is a huge physical and emotional trauma as well as a devastating loss.
What can also change you is the responses of people around you when it happens. 30 years ago, when your friend's baby was stillborn, the procedures around perinatal death will have been very different. She may not have seen her baby, or known whether the baby was buried or cremated. She may have been sedated and told to forget it ever happened. This is what happened to a relative of mine whose newborn daughter died on the late 70s, and it's haunted her for her whole life.
One of the particularly hard things for me when I lost my daughter is that it was impossible to talk about it. My poor baby had died, I was beside myself with grief, but after the first few weeks, if I mentioned it people looked terrified, or thought I was being macabre. To this day if I mention something that happened 'when I was pregnant', I get that frightened look from people.
That's where the shame comes in - there's something about my life that is so awful, and makes people so squeamish, that it has to be hidden. And at the same time, saying I don't have children when acquaintances ask me - just to spare them the awkwardness - feels hugely disloyal to my daughter, and I feel guilty that I'm 'erasing' her. It's very hard to work out what to do.
You can spend 30 years feeling ashamed and guilty, or you can decide you're not going to pretend your firstborn child never existed - and that can make others uncomfortable. But it's really no skin off their nose.
If you're worried your friend is being inappropriate, try to think whether it would be inappropriate to mention a parent or grandparent who had died. Probably not. As a culture we're terrible at talking about death in general, but we're even more terrible at talking about the death of babies. It can only be a good thing to get a bit better at talking about it without all the superstition.
If your friend thinks she might want some more support, as August says, Sands is a really amazing resource - though I would not send your friend to the forum first. Some people use photos of their stillborn babies as profile pics; having not been warned about that beforehand I got an awful shock and haven't been back since. Cruse bereavement charity will work with complex bereavement from a long time ago. Your friend may also want to try the Compassionate Friends, who offer counselling for the loss of a baby or child, whenever it happened.