wow at this thread..
your gf is being overly sensitive, which is understandable. She will get fat, thats pregnancy for you. People making comments arent doing it maliciously in 99.9% of cases and especially in this case as you said she is a friendly person.
the absolute first port of call would be mentioning it to the manager herself. your gf doesnt have to do it, you can. just go have a quiet word and ask her to tone it down as your gf is a bit upset about it. she will probably be horrified that she was causing offence and stop immediately amid floods of apologies.
all this talk of going directly to HR without even mentioning any offence being taken are a sad commentary on todays Britain. :/