Although it is certainly possible to change a child’s name after the fact, perhaps your friend can be persuaded to select a name with which she can be happy if you remind her of all the hassle that she and the child could run into if she does not select a permanent official name at the time of birth.
These are some scenarios that could be problematic:
As skdo23 pointed out, the lack of a name on a birth certificate and something to place into all “the right fields” in a database, she could run into an insurance nightmare. I know this would be a huge hassle in the US, I’m not sure about in the UK with national healthcare.
If a name is selected after the initial birth certificate is issued, the parents and child may have an ongoing hassle explaining the “modified” birth certificate later on.
Two hundred years from now, a descendant of the child may be doing genealogy work and run into a brick wall trying to figure out what happened with that birth certificate–it would probably only be marked “modified,” and there may be no clue whether it was due to a name change, a one- or two-parent adoption, a sex change, etc. I’m a genealogist, so it matters to me… maybe it doesn’t matter to the average person. 
The child may someday run into some arbitrary rule which says, “provide a copy of your original birth certificate,” and a brainless corporate or government drone will hassle him or her over something that could have been avoided. It would probably work out in the end, but as a new mother, I’m sure that your friend will be eager to make her new baby’s life simpler–both now and in the future.
I don’t know how this might work in the UK, but if it were to occur in the US, I could envision a hassle for the child obtaining a copy of the certificate later. It may turn out that modified birth certificates can only be obtained through a central repository, rather than locally, as with an original.
Finally, many of your friend’s friends and family are likely to be obnoxious about their lack of a name for the baby–it’s human nature to want to immediately assign names and labels to things (think of the media hullabaloo over the couples that choose not to disclose their child’s sex). Your friend will make her own life, especially during the trying time of late-night feedings and frequent diaper changes, easier if she just chooses a name in the next few weeks. She can later give the child any nickname she wishes.